Effect of zinc deficiency on hepatic enzymes regulating vitamin A status

The elevation of hepatic vitamin A (VA) in zinc deficiency (ZD) is thought to be due, in part, to a reduced synthesis of plasma retinol-binding protein with a subsequent decrease in the release of retinol into the circulation. We hypothesized that the hepatic VA elevation may also be secondary to a change in the activity of enzymes that either regulate retinol degradation or affect the synthesis or hydrolysis of retinyl esters. To examine this question, 20 rats were divided into two groups and pair-fed for 3 wk. ZD rats received a ZD diet (zinc 2.3 mg/kg diet) and controls were fed a zinc-sufficient diet (zinc 50 mg/kg diet). The enzymes studied were retinyl ester hydrolase (REH) and microsomal acyl coenzyme A:retinol acyl transferase (ARAT), the principal enzymes regulating retinyl ester hydrolysis and synthesis. The activities of retinol (alcohol) dehydrogenase (ADH) and retinal oxidase (RO), enzymes regulating retinol degradation to polar metabolites, were also studied. ZD caused an increase in both total hepatic VA concentration and total hepatic VA content, but did not alter the ratio of retinol to retinyl esters. The specific activities of REH and ARAT were not affected by ZD. However, ZD caused a significant reduction in the activity of ADH, the enzyme that catalyzes the first step in retinol oxidation. In contrast, the activity of RO, the enzyme that regulates the irreversible oxidation of retinal to retinoic acid, was significantly increased in ZD rats. These findings indicate that the elevation in hepatic levels of VA in ZD rats may be, in part, secondary to decreased retinol degradation.