Kinetic studies on the dissociation of adenosine cyclic 3',5'-monophosphate from the regulatory subunit of protein kinase from rabbit skeletal muscle

The exchange rate of unlabeled adenosine 3',5'-monophosphate (cAMP) with labeled [3H]cAMP in the dimeric regulatory subunit-cAMP complex of cAMP-dependent protein kinase, type I, purified from rabbit skeletal muscle is described by using the equilibrium isotope exchange technique. Results indicate that the rate of exchange carried out in the absence of the catalytic subunit (C) is rather slow with a half-life of approximately 870 s. This slow exchange rate is not affected by the presence of MgATP (50 microM). However, when both MgATP (50 microM) and C (1-13 NM) are present, the rate of isotope exchange is observed to increase markedly. Furthermore, less than stoichiometric amounts of C are required for the increase in the rate of cAMP exchange, indicating that the effect of C on the rate enhancement is a catalytic process. These results indicate that in the presence of MgATP, a ternary complex between C and regulatory subunit-cAMP complex must be formed, and a dynamic equilibrium between the eternary complex and its dissociable species must be reached within seconds. On the basis of our kinetic data, it is proposed that the formation of this ternary complex intermediate allows the rapid activation or the inactivation of cAMP-dependent protein kinase following changes in the cellular cAMP levels.

Ambient temperature of hypoxia: a differential study of liver catalase and cytochrome oxidase in guinea pig

The activities of liver catalase and cytochrome oxidase have been determined in sea-level and high-altitude native guinea pigs exposed to different ambient temperatures. The activities of both of these enzymatic systems have been found to increase as ambient temperature is reduced, and this occurs in the sea-level and the high-altitude animals. At equal temperatures, cytochrome oxidase activity is identical in the liver of guinea pigs from sea-level and high-altitude. Catalase activity is approximately 50% lower in the high-altitude animals than in the sea-level ones maintained at the same ambient temperature. It is necessary to reassess current data on hypoxia-induced enzymatic and hormonal changes measured under conditions where the ambient temperature was not controlled, especially in those cases involving volunteer human subjects.

Anoxic cardiac arrest: Its effect on myocardial mitochondrial metabolism

Forty eight healthy dogs are subjects to cardiopulmonary bypass and divide into two groups according to whether the perfusion is performed in normothermia or moderate hypothermia (30 degrees C). The effects on oxidative phosphorylation and mitochondrial respiration, produced by increasing periods of myocardial anoxia from aortic cross clamping, are studied. A clear correlation is found between the disturbances of mitochondrial metabolism and the future cardiac recovery. Hypothermia shows a protective effect on the anoxic myocardium. It is concluded that the period of absolute safety in cardiac anoxia, as far as mitochondrial function is concerned, is 15 minutes in normal thermic perfusion and 90 minutes in moderately hypothermic ones.

Catalysis of ester hydrolysis by mixed micelles containing N-alpha-myristoyl-L-histidine

Model compounds are described for the study of the properties of amino acid side chains in the surface of micelles. Mixed micelles of N-(alpha)- mnyristoyl-L- histidine and cetyltrimethylammonium bromide catalyze the hydrolyses of p-nitrophenyl acetate and p-nitrophenyl caprylate at much higher rates than imidazole or histidinie do. The reaction shows a kinetic behavior similar to that of surface-catalyzed reactions.