Some characteristics of cotton pyrophosphatase activation by magnesium

The effect of Mg²⁺ ions on inducing pyrophosphatase activity of germinating cotton (Gossypium hirsutum L.) seeds was investigated. The presence of Mg²⁺ ions in the germination medium markedly shortened time for the attainment of the pyrophosphatase maximum activity (T _max). In the absence of Mg²⁺ ions in the nutrient medium, T _max comprised 6.0–6.5 days, whereas in the presence of 3–5 mM Mg²⁺, T _max was decreased to 3–4 days. An increase in the concentration of Mg²⁺ ions in the medium up to 5 mM resulted in an increase in pyrophosphatase activity. The effect of Mg²⁺ ions on the activity of a purified pyrophosphatase preparation isolated from three-day-old cotton seedlings was investigated. Mg²⁺ ions did not affect the rate of attainment of a maximum pyrophosphatase activity, but decreased the value of the Michaelis-Menten constant.     

Intrinsic effects of solvent polarity on enzymic activation energies

The effect of organic solvents on subtilisin Carlsberg catalysis has been investigated with the aid of a thermodynamic analysis. Saturation solubility experiments were performed to provide a quantitative measure of substrate desolvation from the reaction medium. This enabled calculation of the intrinsic enzymic activation energy and resulted in a linear free energy relationship with respect to solvent polarity. The results indicate that the intrinsic activation energy of subtilisin catalysis is lowest in polar organic solvents, which may be due to transition state stabilization of the enzyme's polar transition state for transesterification.     

Formation of formaldehyde from S-adenosyl-L-[methyl-3H]methionine during enzymic transmethylation of histamine

The experiments described here show that radiolabelled formaldemethone is formed from the radiolabelled methyl group of S-adenosyl-L-methionine in the course of the enzymic conversion of histamine to N tau-methylhistamine in the presence of dimedone suggesting that the formation of HCHO is probably linked to the enzymic transmethylation of histamine.