Guanine Nucleotide‐Binding Protein Regulation of Microsomal Phospholipase D Activity of Canine Cerebral Cortex

Abstract

The hydrolytic activity of microsomal phospholipase D from canine cerebral cortex was measured by a radiochemical assay using 1,2-dipalmitoyl-sn-glycerol-3-phosphoryl[3H]choline and 1-palmitoyl-2[9,10(n)-3H]palmitoyl-sn-glycerol-3-phosphorylcholine as the exogenous substrates. Of several detergents tested, Triton X-100 was found to be the most effective in allowing expression of phospholipase D hydrolytic activity. The microsomal phospholipase D does not require any metal ion for its hydrolytic activity. Calcium and magnesium were slightly inhibitory between concentrations of 1 and 4 mM, but zinc was greatly inhibitory, causing a loss of > 90% activity at the 4 mM concentration. Nonhydrolyzable guanine nucleotide analogues such as guanosine 5''-(3-O-thio)triphosphate and guanyl-5''-yl-(.beta., .gamma.-methylene)diphosphonate but not guanosine 5''-(2-thio)diphosphate were able persistently to stimulate phospholipase D hydrolytic activity at micromolar concentrations. Guanosine 5''-(2-thio)diphosphate was capable of partially blocking guanosine 5''-(3-O-thio)triphosphate stimulation of phospholipase D. Aluminum fluoride was also able to cause a two- to threefold increase in hydrolytic activity of the phospholipase D. Cholera toxin had a stimulatory effect on the hydrolytic activity of phospholipase D, whereas islet-activating protein pertussis toxin had no effect. These results indicate that regulation of microsomal phosphatidylcholine phospholipase D activity by the guanine nucelotide-binding protein(s) in canine cerebral cortex may play an important role in signal transduction processes as well as in brain choline metabolism.