Retinol 4-hydroxylase

This chapter discusses incubation and chromatographic procedure of retinol 4-hydroxylase. It also discusses the retinol 4-hydroxylase activity in microsomes and reconstituted systems. By revealing that microsomes are capable of actively metabolizing retinol and by elucidating that cytochrome P-450 isozymes are involved, one has acquired a better understanding of the physiological role of a family of microsomal cytochrome P-450 isozymes that appear to be poorly inducible by xenobiotics. It is now increasingly apparent that microsomal cytochromes P-450 play a role not only in the detoxification of foreign compounds but also in important physiological processes, including vitamin A metabolism and maintenance of vitamin A homeostasis. Retinol and retinoic acid have key functions not only in terms of cellular differentiation and maintenance of the normal integrity of mucosal tissues, but also for the prevention of carcinogenesis through various mechanisms, including the inhibition of the microsomal activation of chemical carcinogens. Enzymes such as cytochrome P-450IIC8 may thereby participate in maintaining the delicate balance between those retinol concentrations that promote cellular integrity and oppose the development of cancer and those that cause cellular toxicity.