EFFECT OF CA2-CHANNEL BLOCKERS, CA2+-IONOPHORE AND INCREASED PYRENE EXCIMER FORMATION ON THE MICROSOMAL GLUCOSE-6-PHOSPHATASE()

The dependence of microsomal glucose-6-phosphatase (G-6-Pase) activity on Ca2+ as well as the membrane lipid microviscosity was studied by t he effect of Ca2+-channel blockers (namely verapamil and nifedipine), Ca2+-ionophore, A23187 and pyrene excimer formation. Channel blockers depressed the G-6-Pase and Ca2+-ATPase while the ionophore increased t hese activities. Dimethyl sulfoxide, a known membrane surface active a gent showed no change. Ca2+-uptake into the membrane has expectedly be en lowered by the channel blockers while the ionophores facilitated th e ion flux. Excimer formation of the fluorescent probe, pyrene as an i ndicator of increased membrane fluidity, and microviscosity calculated from there on, showed that Ca2+- and lipid microenvironment in the me mbrane significantly influenced the activity of G-6-Pase. Membrane lip id composition such as phospholipid/cholesterol molar ratio which also indicates an increased membrane fluidity is markedly increased with t he ionophore but decreased with the channel blockers, while protein/ph ospholipid ratio remained unchanged. Microsomal G-6-Pase is a multicom ponent multifunctional protein. It is argued that Ca2+ may play the ro le of an obligatory cofactor not only for the hydrolysis of G-6-P (cat alytic part of the enzyme) but also involved in the regulation of subs trate and product transport in or out of the endoplasmic reticulum lum en.