Schindler_1998_Eur.J.Biochem_251_863

Hepatic retinyl ester hydrolase (REH) activity was isolated from porcine and human liver and characterized, and some of its properties were compared with those of other retinyl-ester-splitting enzymes. Sequence analysis revealed that the REH proteins are structurally similar to non-specific carboxylesterases and distinct from bile salt-activated lipases and cholesterol esterases. Pig REH, a 64-kDa protein, hydrolyzed retinyl palmitate at a rate of 595 nmol x h(-1) x mg(-1) protein in the presence of 100 mM Chaps with an apparent Km value for retinyl palmitate of 27.5 microM. The pH optimum was 7.0-9.2. Its human counterpart has a molecular mass of 65 kDa and a pH optimum near 6.5. In the presence of Chaps, pig REH activity was stimulated up to 1.7-fold by various non-ionic detergents. The ranking order of retinyl palmitate cleavage initiated by the stimuli was n-dodecylglucoside > octanoyl-N-methylglucamide > n-octyglucoside > n-dodecylmaltoside > Triton X-114 > Triton X-100. Porcine REH was effectively inhibited by alpha-tocopherol and bis-(4-nitrophenyl) phosphate [(Np)2P]. The structural, immunological and catalytic features, pH dependence, and the effect of (Np)2P on enzyme activity of pig REH are similar to those reported for the non-specific carboxylesterase ES-4. However, ES-4 differed from REH in molecular mass and the requirement of Chaps or Chaps-like detergents as cofactor. Judging from these results, pig REH may be a non-specific carboxylesterase isoform.