RETINYL ESTERASE-ACTIVITY OF PURIFIED RAT-LIVER RETINYL ESTER LIPOPROTEIN COMPLEX

Retinyl ester lipoprotein complex from rat liver was shown to possess a retinyl esterase activity toward its own ligand complement. In the presence of serum albumin the retinyl esterase activity at 30 °C was about fivefold larger than the activity at 4 °C, while higher temperatures than 30 °C led to some degradation of retinyl compounds. The pH optimum was 7.8. The esterase activity was markedly enhanced by serum albumin although the serum albumin as such had no retinyl esterase activity. In the presence of serum albumin and under optimal conditions, some 75 to 80% of the total retinyl ester complement of the lipoprotein was hydrolyzed in 24 h. The retinyl esterase activity was totally abolished by treatment with the serine esterase inhibitor diisopropyl fluorophosphate (1.4 × 10-4 M), by treatment with sulfhydryl reagents, and by detergents (0.2% of Tween 80 and sodium deoxycholate). From this series of experiments it was concluded that the retinyl ester lipoprotein complex possesses the additional physiological function of hydrolyzing its own retinyl ester complement to unesterified retinol which may then combine with serum retinol-binding protein. © 1979.