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PubMed1968

Enzymatic properties of the inner and outer membranes of rat liver mitochondria.

Treatment of rat liver mitochondria with digitonin followed by differential centrifugation was used to resolve the intramitochondrial localization of both soluble and particulate enzymes. Rat liver mitochondria were separated into three fractions: inner membrane plus matrix, outer membrane, and a soluble fraction containing enzymes localized between the membranes plus some solublized outer membrane. Monoamine oxidase, kynurenine hydroxylase, and rotenone-insensitive NADH-cytochrome c reductase were found primarily in the outer membrane fraction. Succinate-cytochrome c reductase, succinate dehydrogenase, cytochrome oxidase, beta-hydroxybutyrate dehydrogenase, alpha-ketoglutarate dehydrogenase, lipoamide dehydrogenase, NAD- and NADH-isocitrate dehydrogenase, glutamate dehydrogenase, aspartate aminotransferase, and ornithine transcarbamoylase were found in the inner membrane-matrix fraction. Nucleoside diphosphokinase was found in both the outer membrane and soluble fractions; this suggests a dual localization. Adenylate kinase was found entirely in the soluble fraction and was released at a lower digitonin concentration than was the outer membrane; this suggests that this enzyme is localized between the two membranes. The inner membrane-matrix fraction was separated into inner membrane and matrix by treatment with the nonionic detergent Lubrol, and this separation was used as a basis for calculating the relative protein content of the mitochondrial components. The inner membrane-matrix fraction retained a high degree of morphological and biochemical integrity and exhibited a high respiratory rate and respiratory control when assayed in a sucrose-mannitol medium containing EDTA.

Animals↗

PubMed1980

Effects of denervation and simple disuse on rates of oxidation and on activities of four mitochondrial enzymes in type I muscle.

To differentiate the effect of muscle contractile activity from that of motor nerve on oxidative processes in type I muscle, oxidative processes were studied in muscle after immobilization and after denervation. The two processes led to similar atrophy of muscle weight and of the mean diameter of muscle fibers. Disuse of soleus muscle (type I) did not affect rates of oxidation of 14C-labeled substrates although these were reduced by disuse of the vastus lateralis (type II). Disuse of the soleus did not affect activities of several mitochondrial enzymes assayed by histochemical or biochemical methods. However, denervation of the soleus did lead to a fall in metabolic rates and enzyme activities. The activity of 3-hydroxybutyrate dehydrogenase fell more than did the activities of succinic dehydrogenase, lipoamide dehydrogenase, or cytochrome-c oxidase in both homogenates and in mitochondrial fractions. These results suggest nerve may regulate mitochondrial enzymes in type I muscle. The mechanism appears to be different from that which regulates oxidative processes in type II muscle.

Animals↗

PubMed2000

Inactivation of the stress- and starvation-inducible gls24 operon has a pleiotrophic effect on cell morphology, stress sensitivity, and gene expression in Enterococcus faecalis.

Enterococcus faecalis induces the synthesis of at least 42 proteins during 24 h of glucose starvation. Because of its induction during carbohydrate and complete starvation (incubation in tap water) and CdCl(2) and bile salts stresses, one of these proteins (Gls24) was qualified as a "general stress protein" and was analyzed at the molecular level. Its corresponding gene, gls24, seems to be the penultimate gene of an operon composed, altogether, of six open reading frames (ORFs). The ORF preceding gls24 (orf4) showed very strong identity with gls24. The deduced polypeptides of these two genes showed similarity with a 20-kDa hypothetical protein from Lactococcus lactis and an alkaline stress protein from Staphylococcus aureus with no previously known biological significance. Data from the operon sequence and Northern analysis led to the conclusions that (i) gls24 possesses its own promoter which is especially induced at the onset of starvation and (ii) the operon promoter is stress inducible in exponential-phase cells. A mutation in the gls24 gene led to a severe reduction of growth rate and reduction of survival against 0.3% bile salts in the 24-h-starved cells compared to the wild-type strain. Moreover, the chain length of the mutant is significantly reduced during growth. These results argue strongly for a role of the protein Gls24 and/or GlsB in morphological changes and in stress tolerance in E. faecalis. Comparison of two-dimensional protein gels from wild-type cells with those from gls24 mutant cells revealed a pleiotropic effect of the mutation on gene expression. At least nine proteins were present in larger amounts in the mutant. For six of them, the corresponding N-terminal microsequence has been obtained. Three of these sequences map in genes coding for L-lactate dehydrogenase, lipoamide dehydrogenase, and pyruvate decarboxylase, all involved in pyruvate metabolism.

Amino Acid Sequence↗

PubMed1985

Sulfoxide reduction catalyzed by guinea pig liver aldehyde oxidase in combination with one-electron reducing flavoenzymes.

The mechanism of the enhancing effect of methyl viologen (MV) and flavin-adenine dinucleotide (FAD) on sulfoxide reduction which is mediated by a combination of aldehyde oxidase (AO) from guinea pig liver and one-electron reducing flavoenzymes, such as milk xanthine oxidase (XO), was examined. The activity of anaerobic reduction of diphenyl sulfoxide (DPSO) to diphenyl sulfide (DPS) was less than 1 nmol/min/mg protein of AO preparation in a system consisting of hypoxanthine, XO and AO. However, the sulfoxide reduction by this system was enhanced about 6- and 100-fold by the additions of FAD and MV, respectively. In the system containing MV or FAD, other one-electron reducing flavoenzymes such as nicotinamide adenine dinucleotide (reduced form) (NADH) dehydrogenase, lipoamide dehydrogenase and glutathione reductase with an appropriate electron donor, could replace XO. The ability of supplemented flavoenzymes to facilitate DPSO reduction correlated with their abilities to reduce MV and FAD. When AO was omitted from the combined system, no sulfoxide reduction was observed. Stoichiometric study revealed that MV semiquinone and FADH2 were oxidized at ratios of 2 and 1 mol, respectively, per mol of DPS formed. These results indicate that either MV or FAD serves as an electron carrier from the supplemented flavoenzymes to AO, a terminal reductase of sulfoxide.

Aldehyde Oxidoreductases↗

PubMed1987

Chronic Achilles paratenonitis in athletes: a histological and histochemical study.

Pathological alterations of chronic Achilles paratenonitis were studied histologically and histochemically in tissue samples obtained operatively from 16 athletes with this complaint and from 3 control patients. The activities of 11 different enzymes--lactate, succinate, malate, glucose-6-phosphate and glutamate dehydrogenases, lipoamide dehydrogenase and glutathione reductase (NADH2- and NADPH2-diaphorases), acid and alkaline phosphatases, phosphorylase and leucylaminopeptidase--were studied. Pathological findings were located diffusely around the tendon. A slight inflammatory cell reaction was found in all cases. The fatty areolar tissue was clearly thickened and edematous, and showed fibrinous exudations, widespread fat necrosis, considerable connective tissue proliferation and adhesion formation. The blood vessels showed profound degenerative and necrotizing changes. The thin membranes of the paratenon were clearly hypertrophied. Increased enzyme activities were mainly found in the fibroblasts, inflammatory cells and vascular walls. A moderate activity of lysosomal enzymes, an increased activity of enzymes of electron transport, anaerobic glycolysis, pentose phosphate shunt and decreased activity of those of aerobic energy metabolism were found. Simultaneously an increased amount of both neutral and acid mucopolysaccharides and a locally increased amount of elastic fibres were found in the inflamed paratenon. These results indicate that marked metabolic changes occur in paratenonitis, i.e. an increased catabolism and decreased oxygenation of the inflamed areas. The morphological alterations suggest that the gliding function of the paratenon may be impaired.

Achilles Tendon↗

PubMed1984

Complex I binds several mitochondrial NAD-coupled dehydrogenases.

NADH:ubiquinone reductase (complex I) of the mitochondrial inner membrane respiratory chain binds a number of mitochondrial matrix NAD-linked dehydrogenases. These include pyruvate dehydrogenase complex, alpha-ketoglutarate dehydrogenase complex, mitochondrial malate dehydrogenase, and beta-hydroxyacyl-CoA dehydrogenase. No binding was detected between complex I and cytosolic malate dehydrogenase, glutamate dehydrogenase, NAD-isocitrate dehydrogenase, lipoamide dehydrogenase, citrate synthase, or fumarase. The dehydrogenases that bound to complex I did not bind to a preparation of complex II and III, nor did they bind to liposomes. The binding of pyruvate dehydrogenase complex, alpha-ketoglutarate dehydrogenase complex, and mitochondrial malate dehydrogenase to complex I is a saturable process. Based upon the amount of binding observed in these in vitro studies, there is enough inner membrane present in the mitochondria to bind the dehydrogenases in the matrix space. The possible metabolic significance of these interactions is discussed.

3-Hydroxyacyl CoA Dehydrogenases↗

PubMed1996

Bacterial defense against aging: role of the Escherichia coli ArcA regulator in gene expression, readjusted energy flux and survival during stasis.

Using two-dimensional gel electrophoresis and N-terminal amino acid sequencing analysis, we demonstrate that a mutant of the global regulatory protein ArcA fails to decrease the synthesis of the TCA cycle enzymes malate dehydrogenase, isocitrate dehydrogenase, lipoamide dehydrogenase E3 and succinate dehydrogenase in response to stasis, while the increased production of the glycolysis enzymes phosphoglycerate mutase and pyruvate kinase is unaffected. Microcalorimetric and respiratory measurements show that the continued production of TCA cycle enzymes in the (delta)arcA mutant is manifested as an elevated rate of respiration and total metabolic activity during starvation. The (delta)arcA mutant is severely impaired in surviving prolonged periods of exogenous carbon starvation, a phenotype that can be alleviated by overproducing the superoxide dismutase SodA. In addition, flow cytometry demonstrates that starving (delta)arcA mutant cells, in contrast to wild-type cells, fail to perform reductive division, remain large and contain multiple chromosomal copies. We suggest that the ArcA-dependent reduced production of electron donors and the decreased level and activity of the aerobic respiratory apparatus during growth arrest is an integral part of a defense system aimed at avoiding the damaging effects of oxygen radicals and controlling the rate of utilization of endogenous reserves.

Amino Acid Sequence↗

PubMed1980

Bovine kidney pyruvate dehydrogenase complex. Isolation of the component enzymes after limited proteolysis with papain.

1. Bovine kidney pyruvate dehydrogenase multienzyme complex is inactivated rapidly by papain. However, none of the component activities of the complex is destroyed during inactivation of the overall reaction. 2. The core component, lipoate acetyltransferase, is cleaved by papain to give principal fragments with Mr 26,500 and 26,000 (as determined by dodecylsulfate gel electrophoresis). Much more slowly, the alpha chain of the pyruvate dehydrogenase component is attacked. 3. Fragmented lipoate acetyltransferase retains its complete enzymatic activity and remains of high molecular weight. It is unable, however, to bind the other component enzymes, pyruvate dehydrogenase and lipoamide dehydrogenase. Therefore, the multienzyme complex is disassembled when treated with papain. 4. A method is described which allows the rapid and convenient isolation of nicked lipoate acetyltransferase as well as unfragmented pyruvate dehydrogenase and lipoamide dehydrogenase from papain-treated complex under very mild conditions. The two uncleaved component enzymes have identical properties and similar specific activities as enzyme preparations obtained by other, more laborious procedures.

Acetyltransferases↗

PubMed1986

Characterization and immunochemistry of pyruvate dehydrogenase complex of Ascaris muscle.

Pyruvate dehydrogenase complex and lipoamide dehydrogenase were purified from muscle of Ascaris lumbricoides var. suum which contains relatively a large amount of the complex. Molecular weights of three constituent enzymes of Ascaris pyruvate dehydrogenase complex were as follows; alpha- and beta-subunits of pyruvate dehydrogenase were 42,000 and 37,000, respectively, lipoate acetyltransferase was 76,000 and lipoamide dehydrogenase was 56,000. Furthermore, two unknown polypeptides having molecular weight of 46,000 and 41,000 were detected. Anti-Ascaris lipoamide dehydrogenase antibody precipitated three constituent enzymes and two unknown polypeptides, suggesting that lipoamide dehydrogenase not only binds tightly to complex, but also two unknown polypeptides bind tightly to complex.

Animals↗

PubMed1976

Effect of triamcinolone administration on content of flavins in rabbit liver.

Triamcinoline acetonide (10 mg per kg of body weight a day) was administered to rabbit fed on a laboratory chow diet. The content of flavins in liver but not in kidney, muscle and brain started to decrease 24 h after a single dose. The activities of enzymes in the liver were determined: the activities of pyruvate dehydrogenase complex, lipoamide dehydrogenase (NADH:lipoamide oxidoreductase EC 1.6.4.3), NADH dehydrogenase (NADH : (acceptor) oxidoreductase EC 1.6.99.3) and D-amino acid oxidase (D-amino acid: oxygen oxidoreductase (deaminating) EC 1.4.3.3) were decreased but those of succinate dehydrogenase (succinate : (acceptor) oxidoreductase EC 1.3.99.1) and xanthine oxidase (xanthine : oxygen oxidoreductase EC 1.2.3.2) remained unchanged. The activities of enzymes in the kidney, however, remained unchanged except the decrease in the activity of pyruvate dehydrogenase complex.

Animals↗

PubMed1977

Biochemical genetics of the alpha-keto acid dehydrogenase complexes of Escherichia coli K12: isolation and biochemical properties of deletion mutants.

Mutants of Escherichia coli K12 with deletions in the nadC-lpd region of the chromosome were obtained for use in studies on the expression of the ace (pyruvate dehydrogenase complex, specific components) and lpd (lipomide dehydrogenase) genes. These were isolated by selecting spontaneous aroP mutants (lacking the general aromatic amino-acid permease and thus resistant to inhibitory aromatic amino-acid analogues) and screening for auxotrophy due to deletions extending into neighbouring genes. From 2892 isolates tested, the AroP- phenotypes of 2322 were confirmed and, of these, 28 stable and independently-derived auxotrophos were designated as deletion mutants. Six nutritionally-distinct categories were recognized: Nad- (8 strains); Nad-Ace-(7): Nad-'Ace-' (3); Ace- (8); 'Ace-' (I); Lpd-(I). The Ace- phenotypes of four isolates designated 'Ace-' were leaky and enzymological studies confirmed that they had less than 7% of parental pyruvate dehydrogenase complex activity. Enzymological studies showed that the 15 Ace- or Nad-Ace- strains all lacked the pyruvate dehydrogenase complex and pyruvate dehydrogenase (EIp) activities and only three retained detectable dihydrolipoamide acetyltransferase (E2p). The one Lpd- strain lacked pyruvate dehydrogenase, dihydrolipoamide acetyltransferase and lipoamide dehydrogenase (E3) activities as well as the activities of the pyruvate and alpha-ketoglutarate dehydrogenase complexes. The results confirmed the gene order nadC-aroP-aceE-aceF-lpd and indicated that no other essential functions are determined by genes within the nadC-lpd region. Resistance to lactate during growth of pps mutants on acetate was directly related to the specific activity of the pyruvate dehydrogenase complex. None of the deletions promoted the high degree of resistance characteristically associated with constitutive expression of the dehydrogenase complex. Six pps mutants having Ace+ or 'Ace-' phenotypes were more sensitive than the parental strains and expression of their ace operons appeared to be affected; most sensitive were the Ace- strains which lacked pyruvate dehydrogenase complex and phosphoenolpyruvate synthetase activities. The lipoamide dehydrogenase activities of the deletion strains (Lpd+) ranged between 30% and 100% of parental levels indicating that expression of their ace operons appeared to be affected; most sensitive were the Ace- strains which lacked pyruvate dehydrogenase complex and phosphoenolpyruvate synthetase activities. The lipoamide dehydrogenase activities of the deletion strains (Lpd+) ranged between 30% and 100% of parental levels indicating that expression of the lpd gene may be affected by the ace operon but can be independent.

Dihydrolipoamide Dehydrogenase↗

PubMed1986

Resolution of branched-chain oxo acid dehydrogenase complex of Pseudomonas aeruginosa PAO.

Branched-chain oxo acid dehydrogenase was purified from Pseudomonas aeruginosa strain PAO with the objective of resolving the complex into its subunits. The purified complex consisted of four proteins, of Mr 36,000, 42,000, 49,000 and 50,000. The complex was resolved by heat treatment into the 49,000 and 50,000-Mr proteins, which were separated by chromatography on DEAE-Sepharose. The 49,000-Mr protein was identified as the E2 subunit by its ability to catalyse transacylation with a variety of substrates, with dihydrolipoamide as the acceptor. P. aeruginosa, like P. putida, produces two lipoamide dehydrogenases. One, the 50,000-Mr protein, was identified as the specific E3 subunit of branched-chain oxo acid dehydrogenase and had many properties in common with the lipoamide dehydrogenase LPD-val of P. putida. The second lipoamide dehydrogenase had Mr 54,000 and corresponded to the lipoamide dehydrogenase LPD-glc of P. putida. Fragments of C-terminal CNBr peptides of LPD-val from P. putida and P. aeruginosa corresponded closely, with only two amino acid differences over 31 amino acids. A corresponding fragment at the C-terminal end of lipoamide dehydrogenase from Escherichia coli also showed extensive homology. All three peptides had a common segment of eight amino acids, with the sequence TIHAHPTL. This homology was not evident in any other flavoproteins in the Dayhoff data base which suggests that this sequence might be characteristic of lipoamide dehydrogenase.

3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)↗

PubMed1995

Dissociation and reassembly of the dihydrolipoyl transacetylase component of the bovine heart pyruvate dehydrogenase complex.

The catalytic activity and the state of aggregation of the dihydrolipoyl transacetylase-lipoamide dehydrogenase binding protein (E2-E3BP) subcomplex of the bovine heart pyruvate dehydrogenase multienzyme complex were investigated. Treatment of E2-E3BP with the chaotropic salts GndnCl or KSCN led to a rapid decrease in transacetylase activity which was accompanied by a loss of the native quaternary structure, as indicated by changes in the sedimentation properties of the E2-E3BP subcomplex. Reassembly or refolding of dissociated E2-E3BP was achieved for the GndnCl-treated subcomplex using a defined protocol. This reassembly procedure effectively excluded all E3BP from the reassembled oligomeric transacetylase. The reassembled oligomeric E2, free of E3BP, was unable to reconstitute the overall activity of the complex following incubation with pyruvate dehydrogenase (E1) and lipoamide dehydrogenase (E3). In binding studies using radiolabeled components it was demonstrated that the reassembled transacetylase, while retaining its capacity for reductive acetylation and its ability to bind E1, lost its ability to bind E3. The evidence presented in this study indicates that the strong association of E3BP with E2 facilitates the binding of E3, the lipoamide dehydrogenase component, and therefore may have an important role in the assembly and ultimately the catalytic activity of the pyruvate dehydrogenase multienzyme complex.

Acetyltransferases↗

PubMed1975

The pyruvate-dehydrogenase complex from Azotobacter vinelandii.

The pyruvate dehydrogenase complex from Axotobacter vinelandii was isolated in a five-step procedure. The minimum molecular weight of the pure complex is 600,000, as based on an FAD content of 1.6 nmol-mg protein-1. The molecular weight is 1.0-1.2 X 10(6), indicating 1 mole of lipoamide dehydrogenase dimer per complex molecule. Sodium dodecylsulphate gel electrophoretical patterns show that apart from pyruvate dehydrogenase (Mr89,000) and lipoamide dehydrogenase (Mrmonomer 56,000) two active transacetylase isoenzymes are present with molecular weight on the gel 82,000 and 59,000 but probably actually lower. The pure complex has a specific activity of the pyruvate-NAD+ reductase (overall) reaction of 10 units-mg protein-1 at 25 degrees C. The partial reactions have the following specific activities in units-mg protein-1 at 25 degrees C under standard conditions: pyruvate-K3Fe(CN)6 reductase 0.14, transacetylase 3.6 and lipoamide dehydrogenase 2.9. The properties of this complex are compared with those from other sources. NADPH reduced the FAD of lipoamide dehydrogenase as well in the complex as in the free form. NADP+ cannot be used as electron acceptor. Under aerobic conditios pyruvate oxidase reaction, dependent on Mg2+ and thiamine pyrophosphate, converts pyruvate into CO2 and acetate; V is 0.2 mumol 02-min-1-mg-1, Km(pyruvate)0.3 mM. The kinetics of this reaction shows a linear 1/velocity-1/[pyruvate] plot. K3Fe(CN)6 competes with the oxidase reaction. The oxidase activity is stimulated by AMP and sulphate and is inhibited by acetyl-CoA. The partially purified enzyme contains considerable phosphotransacetylase activity. The pure complex does not contain this activity. The physiological significance of this activity is discussed.

Acetyltransferases↗

PubMed1977

Lipoamide dehyrogenase immobilized on porous glass.

Lipoamide dehydrogenase (NADH:lipoamide oxidoreductase, EC 1.6.4.3) isolate from pig heart and Escherichia coli was covalently coupled by both diazonium and amide bonds to controlled pore glass beads (96% silica). When the enzyme was immobilized in the presence of NAD+, the enzyme no longer exhibited its normal requirement for NAD+ for full activity. If the immobilized enzyme was then treated with NADase, the requirement for NAD+ was restored. Enzyme immobilized in the absence of NAD+ exhibited normal NAD+ dependence both prior to an after NADase treatment. These results are discussed in terms of co-immobilization of NAD+ at or near the allosteric site of the enzyme.

Allosteric Site↗

PubMed1998

Assay of protein-bound lipoic acid in tissues by a new enzymatic method.

A new enzymatic method for the determination of protein-bound lipoic acid was established. Bound lipoyl groups were liberated in the form of lipoyllysine by protease digestion and assayed by lipoamide dehydrogenase (NADH:lipoamide oxidoreductase, EC 1.8.1.4)-mediated NADH oxidation. NADH oxidation was coupled to reduction of the lipoyl disulfide group. Fluorescence kinetics of NADH oxidation were markedly enhanced by the addition of glutathione disulfide, recycling the enzyme-mediated lipoyl/dihydrolipoyl conversion. In the presence of a large excess of glutathione disulfide, NADH oxidation follows pseudo-first-order kinetics in terms of lipoyllysine concentration. A good linear correlation is obtained between the oxidation rate and lipoyllysine concentration up to 5 microM and the calibration curve indicates that the detection limit could be 100 nM lipoyllysine. The method was applied to protease lysates of bovine, rat, and rabbit tissues to determine lipoyllysine levels. Kidney and liver were found to have the highest content of lipoyllysine in the range of 3.9 to 4.6 nmol/g rat or rabbit wet tissue or 11.6 to 13.1 nmol/g bovine acetone powder.

Animals↗

PubMed1983

Muscle pyruvate oxidation following denervation and reinnervation.

Muscle pyruvate metabolism was studied in rats following sciatic nerve crush. Control studies showed high pyruvate dehydrogenase and lipoamide dehydrogenase enzyme activity in muscle with type 1 fiber predominance (soleus) and low activities in muscle with type 2 fiber predominance (extensor digitorum longus), whereas lactate dehydrogenase was much higher in the latter. Following denervation, both muscles showed a significant reduction in pyruvate dehydrogenase enzyme activity. During reinnervation, muscle with type 2 predominance developed significantly elevated pyruvate oxidation enzyme activities.

Animals↗

PubMed1984

Aortic enzymes and lactate in high altitude-raised and cholesterol-fed rabbits.

Fourteen male rabbits born at elevation 4000 ft (first experimental series) were transferred at age of 2 months to elevation 12470 ft and raised there for 18 weeks. Half of the animals remained on a commercial rabbit chow (group H) while the other half was on the same diet supplemented with cholesterol (group C). Eight male rabbits raised at sea level served as controls (group S). Intima-media homogenates from the thoracic aortas were assayed for lactate dehydrogenase (LDH), malate dehydrogenase (MDH), lipoamide dehydrogenase, pyruvate kinase (PK), phosphofructokinase (PFK) and the lysosomal hydrolases beta-glucuronidase and N-acetyl-beta-glucosaminidase (NAGA). Aortic lactate and glucose were also measured. Thirty-two male rabbits (second experimental series) were subdivided into 4 groups. Rabbits were fed a cholesterol-supplemented diet not only at high altitude (8 rabbits matching group C) but also 8 animals raised at sea level. The degree of atherosclerosis in the aortas of these 4 groups was assessed by measuring the aortic cholesterol contents. Plasma cholesterol was also determined. In the aortas of the rabbits of group H the activity of PK was significantly elevated, and the activity of the lysosomal hydrolases significantly decreased compared with aortas of group S rabbits. There was no difference in the other enzyme activities or in the aortic glucose and lactate content of these groups. Cholesterol feeding of the animals of group C resulted in a significantly increased activity of the lysosomal hydrolases as well as of LDH and PK. The lipid analyses (second experimental series) revealed a trend to a lower concentration of aortic cholesterol in the high altitude than in the sea level animals, both fed a cholesterol diet, in spite of the higher plasma cholesterol concentrations in the high altitude animals. The low aortic lysosomal hydrolase activities in the high altitude rabbits are in accord with their comparatively lower susceptibility to experimental atherosclerosis. This metabolic feature may be due to a lower degree of exposure of these aortas to injurious factors, such as infections or lower blood pressure. The elevated activity of PK without increased lactate content in group H animals seems to parallel the well-known general adaptation of the organism to high altitude hypoxia, and does not indicate a metabolic switch toward anaerobic glycolysis.

Acetylglucosaminidase↗