Esterification and hydrolysis of vitamin A in the liver of brook trout (Salvelinus fontinalis) and the influence of a coplanar polychlorinated biphenyl

Recent reports of extremely low retinoid stores in fish living in contamina ted river systems prompted an initial investigation of the mechanisms of he patic storage and mobilization in brook trout. Enzyme characterization in m icrosomes revealed a lecithin:retinol acyltransferase activity (LRAT) optim um in the alkaline range (pH 9.0; V-max = 0.6 nmol per mg prot. h(-1); K-m = 10.2 mu M) which is not known to occur in mammals, in addition to a secon dary optimum at pH 6.5 typical of mammals. Acyl CoArretinol acyltransferase (ARAT) kinetic parameters were quite different to those of mammals. The su bstrate affinity of trout ARAT (K-m = 1.6 mu M) was approximately 22-fold g reater than that of the rat while maximal velocity (V-max = 0.2 nmol per mg prot. h(-1)) was 18-fold less. Retinyl ester hydrolase activity (REH) was optimal under acid conditions (pH 4.2; V-max = 6.6 nmol per mg prot. h(-1); K-m = 0.6 mM), was inhibited by a bile salt analogue and was greater in ma les than females. This REH was tentatively categorized as a bile salt-indep endent, acid retinyl ester hydrolase (BSI-AREH). REH was inhibited in a dos e-dependent manner following in vivo exposure to a representative environme ntal contaminant the coplanar polychlorinated biphenyl (PCB), 3,3',4,4'-tet rachlorobiphenyl (TCBP). Inhibition may be an indirect effect because enzym e activity was not affected by in vitro exposure of control microsomes. REH inhibition in the brook trout may affect the uptake of retinyl esters (REs ) from chylomicron remnants as well as the mobilization of stored REs. (C) 1999 Elsevier Science Inc. All rights reserved.