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At least 307 records · Page 17Linked to original sources
PubMed1990

Single-compartment model analysis of thyrotropin-releasing hormone kinetics in hyper- and hypothyroid patients. Kinetic studies using a combined system of RIA and FPLC.

The pharmacokinetics of thyrotropin-releasing hormone (TRH) were assessed following an i.v. injection in blood of ten hyperthyroid, ten hypothyroid, and six normal subjects. A single-compartment model was employed. After methanol extraction, TRH concentrations were analyzed using a specific radioimmunoassay technique combined with fast protein liquid chromatography (FPLC). As for the basal levels of TRH, no differences were observed in either study group. Peak concentrations were always present two min after the injection of TRH. In the euthyroid subjects, TRH blood levels had a half-life (t1/2) of 6.5 +/- 0.41 min, mean +/- SD, while t1/2 was 7.2 +/- 0.62 min in the hyperthyroid and t1/2 was 12 +/- 1.67 min (p less than 0.001) in the hypothyroid patients. The metabolic clearance rate (MCR) (82.2 +/- 15.3 liters/m2/day vs. 89.8 +/- 17.2) and the volume of distribution (Vd) (7.1 +/- 4.2 liters vs. 7.3 +/- 3.4) were approximately the same in the normal subjects and in the hyperthyroid group. MCR (66.2 +/- 15.3 liters/m2/day) and Vd (6.2 +/- 3.3 liters) were found to be lower in the hypothyroid patients. In FPLC, when TRH was added to plasma, it eluted in one peak. Blood samples taken 5 min after TRH i.v. injection had an elution profile of 9.94 ml. These data indicate that 1) TRH has a very short half-life, 2) hypothyroidism can prolong the t1/2 of exogenous TRH, and 3) when TRH should be used in clinical studies, the function of the thyroid gland has to be taken into consideration.

Chromatography, Liquid↗
PubMed1984

Substrate-dependent kinetic behavior of horse plasma cholinesterase: evidence for kinetically distinct populations of active sites.

The inhibition of horse plasma cholinesterase by propranolol showed characteristics which depended upon the identity of the substrate used. With butyrylthiocholine as substrate, the inhibition showed a first-order dependence on inhibitor concentration, and was characterized by a Ki of 8 microM (pH 7.4, 20 degrees C). With p-nitrophenylbutyrate as substrate, a biphasic v-1 versus [I] relationship was obtained. The biphasic curve could be resolved into two components, with apparent Ki's of 9 microM and 1.3 mM. Use of butyrylthiocholine as alternative substrate resulted in partial inhibition of p-nitrophenylbutyrate hydrolysis. Inhibition of butyrylthiocholine hydrolysis by p-nitrophenylbutyrate could be accounted for by pure competitive inhibition at two sites. The results were interpreted in terms of a four-site, low-symmetry model, in which two active sites could process both substrates, and the remaining sites could process only p-nitrophenylbutyrate.

Allosteric Site↗
PubMed1977

Cytochrome f and plastocyanin kinetics in Chlorella pyrenoidosa. II. Reduction kinetics and electric field increase in the 10 ms range.

On dark-adapted Chlorella, after one flash, plastocyanin (PC) undergoes reduction with a half-time of 7 ms. After 4 or 5 flashes, the reduction of PC+ in the 10 ms range is suppressed, and the level of oxidized plastocyanin increases during the next few flashes before reaching a stationary value. Cytochrome f exhibits approximately the same pattern. The reduction of PC+ and cytochrome f+ in the 10 ms range is correlated with an increase of the electrice field named phase b (Joliot, P. and Delosme, R., Biochim. Biophys. Acta 357 (1974) 267-284). Both need the presence of a compound R' in the reduced state. A dark electron transfer involving a carrier of electrons across the membrane, a proton carrier, R' as terminal reducant, PC+ and cytochrome f+ as terminal oxidants, would account for this field generation. Cooperation between the electron transfer chains is implied at the level of plastocyanin oxidation. An equilibrium constant of about 2 is observed between cytochrome f and plastocyanin before 1 ms and after 500 ms after the photochemical reactions. We observe that cytochrome f and plastocyanin are not connected from 1 to 100 ms after a photochemical reaction. The equilibrium constant between plastocyanin and P-700 remains large [20] under these conditions.

Chlorella↗