Effect of barbiturates on polyphosphoinositide biosynthesis and protein kinase C activity in synaptosomes

Previously, it has been found that phenobarbital inhibited protein kinase C (PKC) and the enzymes of the metabolism of polyphosphoinositide, especially phosphatidylinositol 4-phosphate (PIP) kinase (PIP-kinase). As a continuation of these studies, a number of barbiturates (barbituric acid, barbital, butabarbital, pentobarbital, amobarbital, phenobarbital, secobarbital and hexobarbital) were tested for inhibition of these enzymes and also of phosphatidylinositol (PI) kinase (PI-kinase), in a synaptosomal preparation at pH 7.8 from the brain of rat. All compounds, except barbituric acid (and Na-barbital for PI-kinase) inhibited the three kinases. However, PKC was approximately 3–5 fold more sensitive to inhibition by the drugs (measured by Ki values) than PIP-kinase, which was 2- to 4-fold more sensitive than PI-kinase. The inhibitory potency of the drugs increased with their lipophilicity, although to a lesser degree than expected from the differences in partition coefficients; the largest deviation from a positive correlation (i.e. hexobarbital) may be the result of the blockade of an imide (-NH) group at one position of the barbituric ring. Concentrations of drugs (after correction for the greater than normal ionization (pH 7.8) of the drugs in the assays) necessary for half-maximal inhibition were well within, or smaller than, those reported necessary for in vitro blocking of nerves. The possibility, therefore, exists that the physiological effects of the barbiturates are, in part, the result of an inhibition of protein kinase C and PIP-kinase.