Reduction of retinaldehyde bound to cellular retinol-binding protein (type II) by microsomes from rat small intestine

Cellular retinol-binding protein, type II (CRBP (II], an abundant protein of the rat small intestine, has recently been shown to be able to bind retinaldehyde in addition to retinol (MacDonald, P.N., and Ong, D. E. (1987) J. Biol. Chem. 262, 10550-10556). Retinaldehyde is produced in the intestine by oxidative cleavage of beta-carotene. The next step in the intestinal metabolism of vitamin A is the reduction of retinaldehyde to retinol which is then esterified for incorporation into chylomicrons. In the present study retinaldehyde bound to CRBP(II) was found to be available for reduction by microsomal preparations from rat small intestinal mucosa. The microsomal activity was about 8 times greater than the activity observed for an equal amount of cytosolic protein. Retinaldehyde reduction utilized either NADH or NADPH as cofactor, with NADH being slightly more effective. The apparent Km for retinaldehyde-CRBP(II) was 0.5 microM, and the Vmax was approximately 300 pmol/min/mg protein, a rate more than sufficient for the needs of the animal. The product retinol remained complexed to CRBP(II). The microsomal enzyme activity reduced free and bound retinaldehyde to approximately the same extent, although the aldehyde function of retinaldehyde bound to CRBP(II) was less accessible to chemical reducing agents than that of free retinaldehyde. Retinol bound to CRBP(II) could not be oxidized by the microsomal activity in the presence of NAD+, while free retinol or retinol bound to bovine serum albumin was oxidized to retinaldehyde. The more favorable reduction versus oxidation of retinoid bound to CRBP(II) consequently favored the reaction known to be required for the ultimate conversion of beta-carotene to retinyl esters for export from the gut.