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K F Tipton

Publications and source records attributed to K F Tipton.

At least 217 records · Page 12Linked to original sources
PubMed1977

A model for the allosteric regulation of pH-sensitive enzymes.

1. In an enzyme that has two independent binding sites for a ligand, any inhibitor that binds solely to the free enzyme will give rise to positive co-operativity. 2. A model is considered for the allosteric control of enzymes by effectors in which their effects are mediated by ligand-induced perturbations of the ionization constants of a group or groups involved in the binding of substrate to the active site. 3. The model described offers a plausible explanation for the observation that the sigmoidal initial-rate curves reported for some regulatory enzymes are not expressed at all pH values where the enzyme is catalytically active.

Allosteric Regulation↗
PubMed1976

5-Hydroxytryptamine is a substrate for both species of monoamine oxidase in beef heart mitochondria.

The activity of beef heart mitochondrial monoamine oxidase towards 5-hydroxytryptamine (5-HT) is inhibited by the selective inhibitors clorgyline, PCO [5-phenyl-3-(N-cyclopropyl)-ethylamine-1,2,4-oxidiazole] and Deprenyl with a biphasic dependence on the inhibitor concentration. The activities towards tyramine, dopamine and tryptamine were also inhibited in a biphasic manner, but the apparent proportions of the two enzyme species active on dopamine and tryptamine depended on the inhibitor used. Phenethylamine oxidation was inhibited in a monophasic manner suggesting that only a single enzyme species was responsible for the oxidation of this substrate. The biphasic response of 5-HT oxidation to inhibition by clorgyline persisted when functionally competent mitochondria were used and was unaffected by the soluble amine oxidase inhibitors semicarbazine and aminoguanidine. These results indicate that the behaviour of the beef heart enzyme towards selective inhibitors is considerably different from that of any preparations previously studied and suggest that the classification of monoamine oxidase activites into A and B types may be only of limited usefulness.

Amines↗
PubMed1975

The effect of pH on the kinetics of beef-liver fructose bisphosphatase.

1. The kinetics of the reaction catalysed by fructose bisphosphatase have been studied at pH 7.2 and at pH 9.5. The activity of the enzyme was shown to respond sigmoidally to increasing concentrations of free Mg2+ or Mn2+ ions at pH 7.2, whereas the dependence was hyperbolic at pH 9.5. At both pH values the enzyme responded hyperbolically to increasing concentrations of fructose 1,6-bisphosphate, although inhibition was observed at higher concentrations of this substrate. This high substrate inhibition was shown to be partial in nature and the enzyme was found to be more sensitive at pH 7.2 than at pH 9.5. 2. The properties of the enzyme, are consistent with the enzyme obeying either a random-order equilibrium mechanism or a compulsory-order steady-state mechanism in which fructose bisphosphate binds to the enzyme before the cation. 3. Reaction of the enzyme with a four-fold molar excess of p-chloromercuribenzoate caused activation of the enzyme when its activity was assayed in the presence of MN2+ ions but inhibition when Mg2+ ions were used. Higher concentrations of p-chloromercuribenzoate caused inhibition. This activation at low p-chloromercuribenzoate concentrations, and the reaction of 5,5'-dithio-bis(2-nitrobenzoate) with the four thiol groups in the enzyme that reacted rapidly with this reagent, were prevented or slowed by the presence of inhibitory, but not non-inhibitory, concentrations of fructose bisphosphate. After reaction with a four-fold molar excess of p-chloromercuribenzoate the enzyme was no longer sensitive to high substrate inhibition by fructose bisphosphate.

Adenosine Monophosphate↗
PubMed1975

The allosteric properties of beef-liver fructose bisphosphatase.

1. The activity of beef liver fructose bisphosphatase has been shown to respond cooperatively to increasing concentrations of the activating cations Mg2+ and Mn2+. The allosteric inhibitor AMP caused an increase in this cooperativity and a decrease in the apparent affinity of the enzyme for the activating cation. 2. The cooperative response of the enzyme to AMP is similarly increased by increasing cation concentrations with a concomitant decrease in the apparent affinity. 3. Direct binding experiments indicated that in the absence of either Mg2+ or Mn2+ the enzyme bound AMP non-cooperatively up to a maximum of two molecules per molecule of enzyme, a result that is indicative of half-sites reactivity. The binding became increasingly cooperative as the concentration of the activating cation was increased. 4. The substrate fructose bisphosphate had no effect on any of these cooperative responses. 5. These results may be most simply interpreted in terms of concerted model in which the activating cation functions both as an allosteric activator and as an essential cofactor for the reaction.

Adenosine Monophosphate↗
PubMed1975

Rat liver mitochondrial monoamine oxidase. A change in the reaction mechanism on solubilization.

1. The kinetics of benzylamine oxidation by a soluble preparation of rat liver mitochondrial monoamine oxidase were investigated and were shown to conform to adouble-displacement (or Ping Pong) mechanism. 2. The pathway differs in detail from that followed by other amine oxidases, including the membrane-bound enzyme in rat liver mitochondrial outer membranes. 3. It is suggested taht the conformation of the protein in the soluble state differs from that in the membrane-bound state. 4. The full rate equations for this mechanism have been deposited as Supplementary Publication SUP 50039 (5pages) at the British Library (lending Division) (formely the National Lending Library for Science and Technology), Boston Spa, Yorks, LS237BQ, U.K.. from whom copies can be obtained on the terms indicated in Biochem. J (1975) 145,5.

Ammonia↗