Association of acylglyceride and retinyl palmitate hydrolase activities with zinc and copper metalloproteins in a high molecular weight lipid-protein aggregate fraction from chick liver cytosol

Abstract

A lipid-protein aggregate fraction of molecular weight approx. 1.8·10 6 was isolated by gel filtration from chick liver cytosol. This aggregate fraction had a hydrated density range of 1.06–1.13, was 45% lipid, contained zinc and copper and was associated with triolein, phosphatidylcholine and retinyl palmitate hydrolase activity. Hydrolytic activities were stimulated by albumin and cholate, but not by dihydroxy bile acids, and inhibited by serine esterase and sulphydryl inhibitors. Incubation of the aggregate with fatty acid-labelled acylglycerides resulted in protein-binding fatty acid fractions with molecular weights of 150000, 60000, 13000 and approx. 2000. Incubation of the aggregate with [ 3H]retinyl palmitate yielded retinol-containing fractions with molecular weights of 150 000, 60 000 and 15 000. The latter peak appears, on the basis of amino acid composition, to be similar to the cellular retinol-binding protein. In addition, on incubation of the aggregate fraction, zinc and copper peaks are found with molecular weights of 150000, 60000 and 12000–8000. The latter were further purified to yield a copper-rich metalloprotein similar to ‘copper-chelatin’ and a zinc-rich metalloprotein, possibly zinc-metallothionein. Both these metalloprotein fractions had acyl hydrolase activity which was depressed in zinc-depleted animals. This may provide a possible explanation for the documented nutritional interactions between zinc and retinol.