Biomedical subjects

J N Finkelstein

Publications and source records attributed to J N Finkelstein.

At least 73 records · Page 4Linked to original sources

PubMed1989

Stimulated rabbit alveolar macrophages secrete a growth factor for type II pneumocytes.

The type II pneumocyte plays a principle role in the maintenance and repair of the pulmonary alveolar epithelium by increasing its rate of proliferation under conditions of epithelial damage. This investigation examined the role of the alveolar macrophage in the control of type II cell division through its ability to produce specific growth factors when activated in vitro. Type II cells were isolated from adult male rabbits and cultured in the presence of media and matrix that support cell proliferation. Proliferation was assessed by cell counting and pulsing with [3H]thymidine, followed by measurements of labeling index and TCA-insoluble radioactivity. Alveolar macrophages were cultured in serum-free media in the presence of a particulate stimulus. Conditioned media was diluted and added to type II cell cultures. Conditioned media from stimulated macrophage cultures was found to double basal type II cell proliferation, whereas media from unstimulated macrophage cultures had no effect. Macrophage production of type II cell growth-promoting activity was dependent on the concentration of the stimulus and the length of the incubation. Investigation into the identity of the growth-regulating protein established that it is heat labile, insensitive to reduction and acidic conditions, and sensitive to trypsin digestion. Its molecular weight appears to be greater than or equal to 25 kD. Addition of several characterized growth factors to type II cell cultures demonstrated that other known growth-promoting products of macrophages do not act as type II cell growth factors. The evidence presented suggests that in vitro activated alveolar macrophages produce a type II cell growth factor that may play a critical role in mediating repair of the alveolar epithelium.

Animals↗

PubMed1989

Effects of nitrogen dioxide exposure on bronchoalveolar lavage proteins in humans.

Nitrogen dioxide (NO2) is a pollutant of both outdoor and indoor atmospheres that has the potential to alter alveolar epithelial permeability. In order to assess alterations in the protein content of alveolar lining fluid induced by brief (3-h) exposures to low-level NO2, normal volunteers were exposed sequentially to air and NO2, separated by at least 2 wk, in an environmental chamber. Bronchoalveolar lavage (BAL) was performed after exposure. Four experimental protocols were used: (1) continuous 0.60 ppm NO2 with BAL performed 3.5 h after exposure (n = 8), (2) background 0.05 ppm NO2 with three 15-min peaks of 2.0 ppm followed by BAL 3.5 h after exposure (n = 15), (3) continuous 0.60 ppm NO2 with BAL performed 18 h after exposure (n = 8), and (4) continuous 1.5 ppm NO2 with BAL 3.5 h after exposure (n = 15). No changes in lavage fluid levels of total protein or albumin were observed in response to NO2. However, exposure to continuous 0.60 ppm NO2 was associated with increases in lavage fluid levels of the antiprotease alpha-2-macroglobulin (alpha 2M) when assessed 3.5 h after exposure (air versus NO2: 20 +/- 1 versus 29 +/- 2 ng/ml, P = 0.01). No significant changes in levels of alpha 2M in BAL fluid were observed in the other exposure protocols. Lavaged cell numbers, differential counts, and viability were not altered by exposure to the pollutant.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1989

Membrane phospholipid methylation is associated with surfactant secretion in rabbit type II alveolar epithelial cells.

We investigated the involvement of membrane phospholipid methylation in receptor-mediated secretion of surfactant in adult rabbit type II alveolar epithelial cells (type II pneumocyte). Phospholipid methyltransferase activity was found in type II pneumocyte microsomes. Cell cultures of adult rabbit type II pneumocytes were then used to assay methyltransferase activity in the presence of the beta-adrenergic agonist, terbutaline, and the methyltransferase inhibitor, 3-deazaadenosine. Terbutaline predictably stimulated adenylate cyclase activity and surfactant secretion. It was also found to stimulate incorporation of methyl groups into phosphatidylcholine and to increase beta-adrenergic receptor availability as assayed by binding of dihydroalprenolol (DHA). Surfactant secretion, as well as adenylate cyclase activity, were stimulated by terbutaline and were inhibited by 3-deazaadenosine. 3-Deazaadenosine did not inhibit DHA binding. These results suggest that phospholipid methylation plays a role in stimulus-secretion coupling in adult rabbit type II pneumocytes.

Adenylyl Cyclases↗

PubMed1989

Induction of the stress response by isolation of rabbit type II pneumocytes.

Protein synthesis by isolated rabbit type II pneumocytes was evaluated by incorporation of labeled amino acids, gel electrophoresis, and fluorography. Type II alveolar epithelial cells preferentially synthesize a small number of proteins immediately following isolation and simultaneously exhibit a depression of all other protein synthesis. Examination of the time course of the appearance and decline of these proteins showed them to be actively synthesized during the first 14 h after isolation. Decline in their production was accompanied by a gradual enhancement in the synthesis of other proteins. Actin synthesis was not measurable 1 h after isolation, but increased to 9% of total label incorporated by 14 h. This recovery was independent of attachment to an extracellular matrix. Treatment with actinomycin D immediately following cell isolation abolished the synthesis of these proteins, suggesting a requirement for active mRNA production. A subset of these proteins are induced by heat treatment of cultured type II cells. The half-life, intracellular localization, isoelectric points, apparent molecular weights, and heat inducibility of this group of proteins are similar to stress proteins found in other mammalian systems. These stress proteins are unique in that their synthesis is induced by the cell isolation process. Possible mechanisms by which cell isolation may induce the stress response are discussed.

Animals↗

PubMed1989

Effects of exogenous fatty acids and inhibition of de novo fatty acid synthesis on disaturated phosphatidylcholine production by fetal lung cells and adult type II cells.

De novo fatty acid synthesis may be an important source of saturated fatty acids for fetal lung disaturated phosphatidylcholine (DSPC) production. To investigate the roles of de novo fatty acid synthesis and exogenous fatty acids, we incubated dispersed fetal lung cells and freshly isolated adult type II cells with exogenous palmitate and oleate and measured DSPC synthesis. Unlike adult type II cells, fetal lung cells did not increase DSPC synthesis when exogenous palmitate was available; adult type II cells increased DSPC synthesis by 70% in the presence of palmitate. Exogenous oleate decreased DSPC synthesis by 48% in fetal cells but not in adult type II cells. Incubation of fetal lung cells with TOFA [2-furancarboxylate, 5-(tetradecyloxy)-sodium], a metabolic inhibitor of fatty acid synthesis, decreased fatty acid synthesis by 65%. There was a simultaneous 56% inhibition of DSPC production, but no effect on protein, DNA, or glyceride-glycerol production, measured by precursor incorporation. The inhibition of DSPC synthesis associated with TOFA was partially prevented by exogenous palmitate but not oleate. Fetal cells prepared from explants that had been cultured in dexamethasone also had TOFA-associated inhibition of DSPC synthesis that was similar to non-dexamethasone-exposed cells. These studies suggest that under baseline conditions of low fatty acid availability, such as in the fetus, de novo fatty acid synthesis in fetal cells, but not in adult type II cells, provides sufficient saturated fatty acids to support maximal DSPC production. Inhibition of de novo fatty acid synthesis resulting in decreased DSPC production in fetal lung cells in conditions of low fatty acid availability suggests that fatty acid synthesis may be central to maintain DSPC synthesis in the fetus.

Aging↗

PubMed1988

Type II pneumocyte changes during hyperoxic lung injury and recovery.

Adult rabbits exposed to 100% O2 for 64 h and then returned to room air for up to 200 h, develop a lung injury characterized by decreased levels of alveolar surfactant followed by a rebound recovery. In the present study we isolated alveolar type II cells from rabbits at various times during hyperoxic exposure and recovery and measured rates of phosphatidylcholine (PC) synthesis, cellular lipid content, and the specific activity of glycerol 3-phosphate (G-3-P) acyltransferase, an enzyme that catalyzes one of the early reactions in phosphoglyceride biosynthesis. These biochemical parameters were compared with measurements of cell size and cell cycle phase by laser flow cytometry. Results showed that alterations in alveolar phospholipid levels in vivo correlated consistently with cellular lipid metabolic changes measured in isolated type II pneumocytes. In particular, alveolar pneumocytes isolated from lungs of rabbits exposed to 100% O2 for 64 h exhibited a 60% decrease in PC synthesis, cell lipid content, and G-3-P acyltransferase activity. All variables then followed a pattern of recovery to normal and ultimately supranormal levels beginning at approximately 3 days postexposure, at which point there was also a measured increase in the number of type II cells in S phase. These findings suggest that O2-induced changes in type II cell surfactant biosynthesis may account, at least in part, for observed changes in lung phospholipid levels in vivo.

Animals↗

PubMed1987

Increased axonal transport in the rat optic system after systemic exposure to methylmercury: differential effects in local vs systemic exposure conditions.

Axonal transport was studied by several techniques in the optic system of adult female Long-Evans rats following systemic exposure to methylmercury in 5 mM Na2CO3. Control rats were treated with the buffer alone. Four mg Hg/kg body weight for 4-6 days, or for 12 days, induced significant changes in the rate of protein synthesis in the retinal cells and in the rate of propagation of protein-bound radioactivity along the ganglion cell axons. Axonal transport of particulate material in both groups treated with methylmercury increased to a rate of 147 mm/day compared to 93 mm/day in controls. Methylmercury was distributed evenly throughout the retinogeniculate system. No clinical neuropathy was evident in either mercury-treated group. It is proposed that the increased rates of transport may represent an adaptive compensatory response to distal axonopathy caused by methylmercury. To investigate why systemic dosing produced effects opposite to those observed with local application of MeHg, various doses of MeHg were tested in the local and systemic paradigms, including doses which yielded equal concentrations of Hg in the retina. The results indicate that the differential response between the two treatment conditions is not a function of local dose, per se. Local and systemic application produce different dose-effect curves, which do not coincide at any dose.

Animals↗

PubMed1986

Reduction of axonal transport in the rat optic system after direct application of methylmercury.

Fast axonal transport of proteins in the optic nerve and tract was quantified by scintillation counts of protein-bound radioactivity along the visual pathway after an intraocular injection of [3H]proline. In control rats the label traveled at a rate of about 60 mm/day, reaching the optic chiasm at 4 h and the lateral geniculate body at 8 h postinjection. When methylmercury was injected simultaneously with [3H]proline, the label traveled at a rate of about 30 mm/day. At 8 h postinjection, the labeled protein had reached the optic chiasm, but the more distal pathway was unlabeled. The same pattern was observed histologically by emulsion autoradiography of the pathway. Some label was detected in the lateral geniculate of methylmercury-treated animals at 8 h, but this may have resulted from local incorporation, as judged by a similar level of labeling in the contralateral visual pathway. Alternatively, it may be the case that a small fraction of the axons in the treated pathway continued to transport proteins in a normal fashion. The very heavy label observed throughout the pathway in controls was present only in the proximal half of the pathway in methylmercury-treated rats. Methylmercury significantly reduced incorporation of [3H]proline in the rat retina, but this reduction was not as great as the effect in the optic nerve. In contrast, cycloheximide, a potent protein synthesis inhibitor, reduced labeled protein in the optic nerve only to the same extent as it reduced incorporation. These results suggest that methylmercury's effect on transport is not dependent solely on its effects on protein synthesis, but represents a separate mechanism of neurotoxicity.

Animals↗

PubMed1986

Insulin receptors and insulin effects on type II alveolar epithelial cells.

Type II alveolar epithelial cells (pneumocytes) were isolated to purity from adult rabbits and analyzed for the presence of cell surface insulin receptors and for effects of insulin on cells. Assays were performed on cells cultured for 24 h in Eagle's minimum essential medium. Insulin binding to cells in culture approached a steady-state level by 180 min at 15 degrees C and remained constant for at least 1 h. Competition experiments using native insulin, proinsulin and desoctapeptide supported specificity of binding. Scatchard analysis of binding revealed a class of high-affinity receptors with Kd = 1.5 X 10(-10) M and a low-affinity component with Kd = 4 X 10(-9) M. The number of receptors was estimated at 2000-4000/cell. Insulin added to cell cultures of type II pneumocytes in concentrations from 5 X 10(-11) to 5 X 10(-8) M resulted in a dose-related increase in uptake of 2-deoxyglucose by cells. Insulin also stimulated the incorporation of choline and glucose into phosphatidylcholine and disaturated phosphatidylcholine.

Animals↗

PubMed1986

A quantitative study of the development of type II pneumocytes in fetal lung.

Cell populations dissociated from fetal rabbit lungs were analyzed by laser flow cytometry for the presence of type II pneumocytes. These cells are distinguishable by the staining of their lamellar bodies with the fluorescent lipophilic dye, phosphine-3R and by their intensity of low-angle light scatter. Lung cells were obtained by enzymatic dissociation from fetal rabbits at gestational ages of 24 d, 27 d, and from 2-d newborn rabbits. Flow cytometric analysis was sufficiently sensitive to discriminate between fetuses. Quantitative analysis of type II pneumocytes showed that newborn rabbits had a distinct cell subpopulation in a region of low-angle light scatter and phosphine-3R fluorescence intensity similar to that previously reported on type II cells from adult rabbits. By contrast, 24-d gestation rabbits had a negligible type II cell subpopulation. Fetuses of 27 and 30 d gestation showed a slow but progressive increase in the numbers of cells in the type II region. Mathematical analyses of light scatter and fluorescence intensity distributions were used to define statistically significant (P less than .05) boundaries that characterize the development of the type II cell subpopulation in fetal rabbit lung. The methods employed offer new possibilities for quantification of developing lung cell subpopulations of particular interest to the problem of respiratory distress syndrome in human neonates.

Age Factors↗

PubMed1986

An examination of the oxidation of mercury vapor by rat brain homogenate.

The oxidation of mercury vapor (Hg degrees) to divalent inorganic mercury (Hg2+) was studied in rat brain homogenates. By using a "degassing" method, it was possible to speciate the mercury present in the homogenate and, for the first time, to measure the rate of oxidation as a function of the substrate (Hg degrees) concentration. Mercury oxidation was first-order with respect to substrate concentration at all concentrations tested, and the first-order rate constant for the oxidation process was proportional to homogenate concentration. The role of catalase compound I in mercury vapor oxidation by brain homogenate was examined by observing the effects of two inhibitors of catalase (catalase compound I) on homogenate mercury-oxidizing activity and catalase activity. Sodium azide (50 mM) completely inhibited both mercury-oxidizing activity and catalase activity. Aminotriazole (3-amino-1H-1,2,4-triazole) (50 mM) completely inhibited only mercury-oxidizing activity; some residual catalase activity was found in the aminotriazole-treated homogenate. It was concluded that catalase compound I plays a major role in the oxidation of Hg degrees, but the possibility that catalase-independent pathways make a minor contribution cannot be excluded.

Amitrole↗

PubMed1986

Effects of methylmercuric chloride, cycloheximide, and colchicine on the reaggregation of dissociated mouse cerebellar cells.

High fetal and neonatal brain levels of methyl mercury (MeHg) have been associated with the abnormal migration of neurons within the cerebellar and cerebral cortices. How MeHg interferes with cellular proliferation, migration, and differentiation is poorly understood. In this study, a cell recognition/cohesion assay based on the ability of dissociated neonatal mouse cerebellar cells to reaggregate was used to test whether MeHg exposure could disrupt cell surface recognition. Reaggregation of dissociated cells was monitored by measuring diameters from low-power photomicrographs. Exposure to 4 mg/kg body wt methylmercuric chloride (MeHgCl) 24 hr prior to the isolation of 3 days postnatal cerebellar cells altered the pattern of reaggregate growth. Between 25 and 51 hr in vitro (hiv), the exposed reaggregates grew at a faster rate than controls. Freshly isolated cells exposed in vitro to 0, 0.5, 1.0, and 4.0 microM MeHgCl initially exhibited a dose-related inhibition in reaggregate growth with an IC50 of 1.5 microM at 24 hiv. Following initial inhibition, exposed groups showed a dose-dependent acceleration in reaggregation similar to that found following in vivo exposure. In contrast, in vitro exposure to cycloheximide resulted in only a dose-related inhibition of reaggregation. No acceleration in growth rate was seen. Colchicine exposure produced no initial inhibition but appeared to mimic the long-term effects seen with both in vivo and in vitro MeHgCl exposure. These studies suggest that MeHgCl alters cerebellar cell recognition through a complex mechanism initially involving depressed synthesis of specific proteins followed by alterations in microtubules. Both effects may involve a disruption in the arrangement of specific cell surface recognition molecules.

Animals↗

PubMed1986

Predictive biochemical assays for late radiation effects.

Surfactant precursors or other products of Type II pneumocytes have the potential to be the first biochemical marker for late radiation effects. This is particularly clinically important in the combined modality era because of the frequent occurrence of pneumonitis and pulmonary fibrosis secondary to radiation or chemotherapy. Accordingly, correlative studies have been pursued with the Type II pneumocyte as a beginning point to understand the complex pathophysiology of radiation pneumonitis and fibrosis. From our ultrastructural and biochemical studies, it is evident that Type II pneumocytes are an early target of radiation and the release of surfactant into the alveolus shortly after exposure persists for days and weeks. Through the use of lavaging techniques, alveolar surfactant has been elevated after pulmonary irradiation. In three murine strains and in the rabbit, there is a strong correlation with surfactant release at 7 and/or 28 days in vivo with later lethality in months. In vitro studies using cultures of type II pneumocytes also demonstrate dose response and tolerance factors that are comparable to the in vivo small and large animal diagnostic models. New markers are being developed to serve as a predictive index for later lethal pneumonopathies. With the development of these techniques, the search for early biochemical markers in man have been undertaken. Through the use of biochemical, histological, and ultrastructural techniques, a causal relationship between radiation effects on type II pneumocytes, pulmonary cells, endothelial cells of blood vessels, and their roles in the production of pneumonitis and fibrosis will evolve.

Animals↗

PubMed1986

Adsorption of natural lung surfactant and phospholipid extracts related to tubular myelin formation.

The microstructure of aqueous dispersions of pulmonary surfactant lipids extracted from bovine lung lavage was investigated by electron microscopic analysis. Following organic solvent extraction (chloroform/methanol) from natural lung surfactant, the mixed lipids (CLL), with 1% residual protein, were dispersed in water by two techniques, probe sonication at 4 degrees C and mechanical vortexing. Surface pressure-time adsorption isotherms were defined for the CLL dispersions, followed by staining with tannic acid, uranyl acetate, and lead citrate for electron microscopy of microstructure. CLL extract dispersions adsorbed in seconds to surface pressures near 45 dynes/cm (surface tension 25 dynes/cm) at low concentrations less than or equal to 0.25 mg/ml after dispersion by sonication of 4 degrees C and by mechanical vortexing. Ultrastructurally, the CLL dispersions were somewhat heterogeneous, but large thin-walled phospholipid vesicles, both intact and fragmented, predominated. No tubular myelin was formed. By contrast, natural lung surfactant (LS) from bronchoalveolar lavage had characteristic regions of tubular myelin when it adsorbed well at low concentrations (less than or equal to 0.25 mg phospholipid/ml). When divalent cations were removed from solution with 5 mM EDTA, this distinctive microstructure was not present and natural LS adsorbed less rapidly; higher concentrations of 0.63 mg phospholipid/ml were necessary for maximal adsorption of natural LS without tubular myelin. These results suggest that while tubular myelin is associated with optimal adsorption for natural LS, it is not a required configuration for rapid adsorption facility at low phospholipid concentrations in general. Specifically, for dispersions of surfactant extracts, other microstructures allow adsorption facility equivalent to that of natural LS with tubular myelin.

Adsorption↗

PubMed1986

In vivo and in vitro demonstration of reduced myelin synthesis following early postnatal exposure to ionizing radiation in the rat.

Irradiation of the immature central nervous system has been demonstrated histopathologically to result in a reduction in the quantity of myelin seen at later developmental ages [S. A. Gilmore, J. Neuropathol. Exp. Neurol. 22, 294-301 (1963). J. A. Beal and J. L. Hall, J. Neuropathol. Exp. Neurol. 33, 128-143 (1974)]. The results from our investigation indicate that this reduction in myelin content can be attributed to a decrease in sulfatide synthesis. Rats received whole-brain irradiation with 0, 500, 1500, 2000, or 2500 rad at 4 days postnatal (dpn). All of the rats exposed to 2000 or 2500 rad and 70% of those exposed to 1500 rad died within 6 to 10 days. At 17 dpn, animals received single intraperitoneal injections of [35S]sodium sulfate. Myelin synthesis, as indexed by the incorporation of sulfate into total lipids and glycolipids, was reduced in a dose-related fashion. To demonstrate a direct effect of ionizing radiation on myelinogenesis, brain cell reaggregate cultures derived from fetal rats were exposed at 12 days in vitro (div) to 0, 250, 500, 1000, or 1500 rad. A dose-related reduction in [35S]sulfate incorporation through 21 div was demonstrated. Reaggregates exposed to 250 or 500 rad but not 1000 or 1500 rad resumed normal myelin synthesis by 28 div. These changes occurred in the absence of histopathological changes, changes in protein content, and changes in the rate of protein synthesis.

Animals↗

PubMed1985

Surface property changes from interactions of albumin with natural lung surfactant and extracted lung lipids.

These experiments characterize the effects of albumin on the dynamic surface activity of natural lung surfactant (LS), and an extracted mixed lipid fraction (CLL), at physiologic temperature, humidity, and film cycling rate on an oscillating bubble apparatus. Measurements of albumin effects on the surface pressure-time (pi-t) adsorption isotherms of CLL and LS are also reported. Results show that albumin in concentrations greater than or equal to 20 mg/ml increased the minimum dynamic surface tension of LS suspensions (0.4 mg phospholipid/ml) from less than 1 dyne/cm to 21 dynes/cm at 37 degrees C. Albumin in low concentrations (2 mg/ml) had a similar detrimental effect on the dynamic surface activity of extracted surfactant lipids, CLL. In addition, albumin also inhibited the isolated adsorption facility of LS and CLL; instead of adsorbing rapidly to their maximum spreading pressures of 45 dynes/cm, both surfactant mixtures (at 0.063 and 0.125 mg phospholipid/ml) adsorbed more slowly or reached lower final surface pressures in the presence of plasma protein. A striking finding was that albumin inhibition of surface activity was moderated or abolished at high lipid concentrations. For example, minimum dynamic surface tensions less than 1 dyne/cm were reached on the oscillating bubble for natural LS at concentrations greater than 0.75 mg/ml and CLL at concentrations greater than 1.5 mg/ml, even in the presence of very large amounts of albumin (100 mg/ml). Similarly, LS and CLL adsorption facility was protected from albumin inhibition at sufficiently high phospholipid concentrations. Albumin inhibition of natural LS adsorption was also moderated by the presence of 1.4 mM Ca2+ ions. These results show that albumin in plasma transudates has the potential to seriously impair alveolar surfactant activity in vivo. However, the detrimental effect will be mitigated if a critical threshold of phospholipid is present.

Adsorption↗

PubMed1985

Dexamethasone increases de novo fatty acid synthesis in fetal rabbit lung explants.

The effects of dexamethasone on pulmonary de novo fatty acid synthesis were investigated in a fetal rabbit lung explant model. Culture of the explants for 2 or 6 days with dexamethasone produced a 48% increase in de novo fatty acid synthesis, measured by 3H2O incorporation, and a 2.5-fold increase in [3H-methyl] choline incorporation into phosphatidylcholine. A dose-response study showed this effect plateaued at dexamethasone concentrations above 10(-9) M. The distribution of the products of de novo fatty acid synthesis was altered by dexamethasone. Treated cultures had a 2.5-fold increase in esterification of the newly synthesized fatty acids to phosphatidylcholine and an increase in the proportion of all newly synthesized fatty acids that were used for phosphatidylcholine synthesis. Dexamethasone treatment significantly increased the total fatty acid content and the phosphatidylcholine fatty acid content of the explants by 27 and 85%, respectively. The newly synthesized fatty acids were esterified almost equally to the 1 and 2 positions of phosphatidylcholine; this distribution was not altered by dexamethasone. These studies demonstrate that fatty acid synthesis and distribution are regulated in the fetal lung, at least in part, by glucocorticoids. In addition, coordinate regulation of fetal lung de novo fatty acid synthesis and surfactant phosphatidylcholine production by glucocorticoids is likely.

Animals↗

PubMed1985

The effect of lateral mobility on the binding and reaction rate at membrane sites.

The rates of membrane processes like transport, hormone action and enzyme reaction depend on the prevailing temperature. Arrhenius plots of such rates show a break at the phase transition temperature. We have very little insight into the molecular mechanism of the effect of phase transition on the membrane phenomena. In general above this temperature there is an onset of two dimensional mobility of the surface sites. Here, we point out that the binding of ligands to these sites should then be described by an adsorption isotherm appropriate for mobile rather than fixed sites. This introduces an additional factor in the rate equation, which correlates well with the observed changes in the rates at membrane sites on gel to liquid crystalline transition.

Enzymes↗