Kinetics and mechanism of acetohydroxy acid synthase isozyme III from Escherichia coli

Acetohydroxy acid synthase (AHAS, EC 4.1.3.18) isozyme III from Escherichia coli has been studied in steady-state kinetic experiments in which the rates of formation of acetolactate (AL) and acetohydroxybutyrate (AHB) have been determined simultaneously. The ratio between the rates of production of the two alternative products and the concentrations of the substrates pyruvate and 2-ketobutyrate (2KB) leading to them, R, VAHB/VAL = R[( 2KB]/[pyruvate]), was found to be 40 +/- 3 under a wide variety of conditions. Because pyruvate is a common substrate in the reactions leading to both products and competes with 2-ketobutyrate to determine whether AL or AHB is formed, steady-state kinetic studies are unusually informative for this enzyme. At a given pyruvate concentration, the sum of the rates of formation of AL and AHB was nearly independent of the 2-ketobutyrate concentration. On the basis of these results, a mechanism is proposed for the enzyme that involves irreversible and rate-determining reaction of pyruvate, at a site which accepts 2-ketobutyrate poorly, if at all, to form an intermediate common to all the reactions. In the second phase of the reaction, various 2-keto acids can compete for this intermediate to form the respective acetohydroxy acids. 2-Keto acids other than the natural substrates pyruvate and 2-ketobutyrate may also compete, to a greater or lesser extent, in the second phase of the reaction to yield alternative products, e.g., 2-ketovalerate is preferred by about 2.5-fold over pyruvate. However, the presence of an additional keto acid does not affect the relative specificity of the enzyme for pyruvate and 2-ketobutyrate; this further supports the proposed mechanism. The substrate specificity in the second phase is an intrinsic property of the enzyme, unaffected by pH or feedback inhibitors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Results of the study of preventive effectiveness and side effects of anti-influenza lactoglobulin in controlled epidemiologic trial]

Prophylactic activity and side action of antiinfluenza lactoglobulin were studied under conditions of controlled epidemiological trial in 6361 persons during the epidemiological rise of influenza incidence. The data obtained pointed to the absence of any protective effect of antiinfluenza lactoglobulin. Marked allergenic properties of lactoglobulin and development of sensitization in the persons under observation were found during the study of the side-effect of the preparation. Thus, the absence of prophylactic efficacy and the sensitizing action of antiinfluenza lactoglobulin pointed to the inexpediency of its use for prophylactic and therapeutic purpose.     

Electrophysiologic effects of serotonin on atrioventricular conduction in dogs

In 12 anaesthetized dogs acute effects of serotonin on the atrioventricular conduction system were studied by obtaining intracardiac electrograms. The significant increase in the heart rate and decrease in aortic pressure elicited by an intravenous infusion of 50 mg/kg.min serotonin for 30 minutes were comparable to those observed in previous investigations. Our data indicate a serotonin induced acceleration of the sinus node pacemaker and atrioventricular conduction by direct and indirect mechanisms.     

Dependence of hypoxic changes in macro- and micro-hemodynamics on the activity of adrenoreceptors in the skeletal muscle in hypothermia

Dependence of hypoxic changes of macro- and microcirculation on the activity of adrenergic receptors in the cooled organism was studied on decentralized shank of cat under perfusion with constant blood flow. After cooling of cat (to 30 degrees C) and blockade of alpha-adrenoreceptors hypoxic hypoxia (10% O2 in N2) caused (a) much greater reduction of precapillary resistance of shank, (b) more striking (by 3 times) increase of capillary filtration coefficient and (c) the increase of capillary pressure and postcapillary resistance in contrast to their decrease to hypoxia under hypothermia before alpha-blockade. Beta-adrenoreceptor blockade had no influence on the changes of the resistance and exchange function of skeletal muscle blood vessels evoked by hypoxia under cooling.     

Enzymes of cyclic nucleotide degradation in eye tissues during experimental ocular herpes

Activity of phosphodiesterases disintegrating cAMP and cGMP in the cornea, sclera and ciliary body was investigated in health and in different stages of experimental herpetic keratitis. The problems concerning the role of the cyclase system in the pathogenesis of herpetic keratitis and the possibility of applying some of the drugs to the disease treatment are discussed.