Liver response tests. VIII. factors influencing the activities of liver-microsomal phosphatases

Abstract

A study has been made of the effect of coumarin and carbon tetrachloride on rat-liver microsomal phosphatases and of the phospholipid requirement for the activity of these enzymes. Coumarin (200 mg/kg/day given orally for 7 days) or carbon tetrachloride (0.5 ml/kg as a single subcutaneous dose) brought about a significant increase in the activities of nucleoside diphosphatases (namely inosine, guanosine, uridine and cytidine) and thiamine pyrophosphatase; adenosine diphosphatase was not affected. Phospholipase A, C or D incubated with liver microsomes in vitro increased the activities of nucleoside diphosphatases and reduced glucose 6-phosphatase and inorganic pyrophosphatase as the hepatotoxins had done in vivo. Whereas the addition of phospholipid has no effect on the raised nucleoside diphosphatase activity, the reduction of glucose 6-phosphatase or inorganic pyrophosphatase was partially returned to the control level by phospholipid. Phospholipids from rat-liver microsomes when added to rat-liver microsomes slightly increased the activity of nucleoside diphosphatases and reduced the activity of glucose 6-phosphatase and inorganic pyrophosphatase. Ox-brain and egg lecithin were more effective in eliciting these enzyme changes. Daily intraperitoneal doses of 10 mg triamcinolone/kg to rats for 7 days or overnight starvation increased the activity of glucose 6-phosphatase and inorganic pyrophosphatase but had no effect on nucleoside diphosphatases. Treatment of microsomal preparations of untreated rats in vitro with triton X-100 increased the activities of glucose 6-phosphatase and pyrophosphatase to the levels attained with triamcinolone or fasting which were not further raised in vitro by triton X-100. The reduced activities of these enzymes resulting from the administration of coumarin or carbon tetrachloride were similarly raised to the same high level by triton X-100 in vitro. Both the enhanced activities of nucleoside diphosphatases brought about by the hepatotoxins and the activities of nucleoside diphosphatases of untreated rats were raised to the same level by triton X-100 in vitro. These results provide evidence for the importance of a phospholipid-protein association in maintaining the activity of rat-liver microsomal phosphatases. It would appear that hepatotoxic compounds, fasting and triamcinolone treatment exert their action on this association rather than inactivating the enzyme or enhancing de novo protein synthesis.