The mechanism of increase in the ATPase activity of sarcoplasmic reticulum vesicles treated with n-alcohols

Abstract

Treatment of sarcoplasmic reticulum vesicles with aqueous n-alcohols caused inhibition of calcium uptake and enhancement of ATPase activity. With increasing alcohol concentration, the ATPase activity reached a maximum (in the case of n-butanol, at about 350 mM) and then decreased. The effect of n-butanol was extensively studied. The purified ATPase enzyme and leaky vesicles treated with Triton X-100 or phospholipase A showed high ATPase activity in the absence of n-butanol. With increasing n-butanol concentration, their atpase activities began to decrease above about 250 mM n-butanol, without any enhancement. In the presence of ATP, the turnover rate of calcium after calcium accumulation had reached a steady level was the same as that at the initial uptake. n-Butanol did not affect these rates. Kinetic analyses of these experiments were carried out. The mechanisms of calcium transport and of increase of ATPase activity in the presence of alcohol were interpreted as follows. After calcium accumulation had reached a steady level, fast influx and efflux continued; the influx was coupled with phosphorylated enzyme (E-P) formation and most of the efflux was coupled with rephosphorylation of ATP from ADP and E-P. The observed ATPase activity is the difference between these two reactions. If alcohol molecules make the vesicles leaky, calcium ions will flow out without ATP synthesis and the apparent ATPase activity will increase. The effect of alcohols on sarcoplasmic reticulum vesicles was separated into two actions. The enhancement of ATPase activity was attributed to a leakage of calcium ions from the vesicles, while the decrease of ATPase activity at higher concentrations of alcohols was attributed to denaturation of the ATPase enzyme itself. The two effects were interpreted in terms of equilibrium binding of alcohol molecules to two different sites of the vesicles; leakage and denaturation sites. Similar analysis was carried out for various n-alcohols from methanol to n-heptanol. The apparent free energies of binding of the methylene groups of n-alcohols were evaluated to be -863 cal/mol for the leakage site, and -732 cal/mol for the denaturation site.