Effects of heating on the ion-gating function and structural domains of the acetylcholine receptor

The ion-gating ability and the protein electrophoretic band patterns of the acetylcholine receptor from Torpedo californica electroplax were examined after receptor-enriched membrane vesicles were progressively heated. The ion translocation function was lost over a temperature range of 40-55 degrees C. Previous results have shown that the stoichiometry of alpha-bungarotoxin binding is not affected by these temperatures, although bound toxin reversibly dissociates within this temperature range, and that toxin binding is irreversibly lost at somewhat higher temperatures [Soler, G., Farach, M.C., Farach, H. A., Jr., Mattingly, J.R., Jr., & Martinez-Carrion, M. (1983) Arch. Biochem. Biophys. 225, 872]. Thermal gel analysis [Lysko, K. A., Carlson, R., Taverna, R., Snow, J., & Brandts, J.F. (1981) Biochemistry 20, 5570], a sodium dodecyl sulfate-polyacrylamide gel electrophoretic procedure which detects thermally induced aggregation of the components of multimeric systems, was applied to heated acetylcholine receptor enriched membranes. This technique suggests two structural domains susceptible to thermal perturbation within the receptor molecule, one consisting of the Mr 50 000 and the two Mr 40 000 subunits and the other consisting of the Mr 60 000 and 65 000 subunits. Heat disrupts molecular events linking agonist binding with ion-channel opening in the acetylcholine receptor molecule.

Reductive methylation as a probe of the heat-labile alpha-bungarotoxin-acetylcholine receptor membrane complex: evidence for surface interactions

alpha-Bungarotoxin (alpha-Bgt) is a potent postsynaptic neurotoxin which blocks neurotransmission by binding very tightly to the acetylcholine-receptor (AcChR) protein. We have previously shown (P. Calvo-Fernandez, and M. Martinez-Carrion (1981) Arch. Biochem. Biophys. 208, 154-159) that alpha-Bgt free in its native solution conformation incorporates 12 methyl groups when reductively methylated using formaldehyde and sodium cyanoborohydride. We now show that when the alpha-Bgt molecule is bound to the AcChR contained in native membranes prepared from Torpedo californica electroplax, the number of accessible methylation sites is significantly reduced. This favors a model of alpha-Bgt-AcChR interaction involving significant numbers of lysyl moieties distributed over a reasonably large surface of the toxin molecule. In addition, this paper presents a novel procedure for the rapid and nondestructive dissociation of the toxin-AcChR membrane complex which takes advantage of the thermal instability of the complex.

Nutritional regulation of glycolysis in rainbow trout (Salmo gairdneri R.)

1. Some physico-chemical constants and the nutritional regulation of pyruvate kinase (PK), phosphofructokinase (PFK) and hexokinase (HK) from rainbow trout liver was investigated. 2. The maximum activity pH for the three enzymes appears to be in a physiological range. 3. The PK-enzyme shows sigmoid kinetic with respect to PEP with a Hill-coefficient of 3.1; the other two enzymes show michaelian kinetic for their substrates. 4. The nutritional treatments show that HK-enzyme increases its level with high carbohydrate diet and decreases with high protein diet and starvation. 5. PFK-enzyme decreases with high protein diet and starvation. 6. PK-enzyme only shows a decrease in level with starvation conditions.

[Physico-chemical properties of guinea pig liver argininosuccinate synthetase (author's transl)]

The behaviour of guinea pig liver argininosuccinate synthetase with respect to pH, temperature and kinetic parameters of substrates and inhibitors has been investigated. The enzyme shows the maximum activity at pH 8 and maximum stability (15 min at 30 degrees C) at pH in the range of 6.5 and 8. With respect to temperature it remains stable (15 min at pH 8) up to 40 degrees C. The results found for the Km values of the enzyme substrates L-citrulline, L-aspartate and ATP are 0.038 mM, 0.045 mM and 0.090 mM respectively. L-ornithine, diaminopimelic acid, pyrophosphate and ATP acted as inhibitors of the enzyme (competitive or not, according to the case). These kinetic studies of substrates and inhibitors were carried out with a partially purified fraction of the enzyme which had been purified 7.3 fold, which practically suppressed the ATP-ase activity, present in crude extracts.

[Physico-chemical properties of guinea pig liver arginase (author's transl)]

Arginase (E.C. 3.5.3.1) the enzyme which catalyses the hydrolysis of arginine to ornithine and urea has been investigated in guinea pig liver in relation to the kinetic constants of its substrate and inhibitors as well as to other physico-chemical properties. The results show that the enzyme has Km value of 19.6 mM for its substrate L-arginine and is competitively inhibited by one of its reaction products, the L-ornithine, and also by L-lysine and diaminopymelic acid. Optimal activity of the enzyme occurs at 10.5 pH and maximal stability in the range of 6.5 to 7.5 pH. The mentioned arginase exhibits temperature dependent activity and stability, being 64 degrees C (15 min and pH 7.5) the half-inactivation temperature. An increase in the activity and temperature stability of the enzyme, when previously activated by heating for 5 min at 45 degrees C in the presence of 10 mM MnCl2, has been achieved.

[The influence of fasting and cortisol upon arginosuccinate synthetase, arginosuccinase and arginase levels in guinea pig liver (author's transl)]

The influence of fasting and cortisol treatment on the levels of arginosuccinate synthetase (E.C. 6.3.4.5.), ARGINOSUCCINASe (E.C. 4.3.2.1) and arginase (C.E. 3.5.3.1) from guinea pig liver has been investigated. The results show a general increase in the levels of the mentioned enzymes, with specific enzyme synthesis for fasting, and unspecific for cortisol treatment.