Modification of drug biotransformation by vitamin C in man

Abstract

VITAMIN C (ascorbic acid) is taken by many individuals in very large doses and there has been some concern about possible adverse effects of such regimens. It has been found that vitamin C is metabolised in part to ascorbic acid sulphate in man¹. Sulphate formation is also an important pathway for the biotransformation of phenolic drugs². This pathway is, however, of limited capacity in man³ (and in many species of animals) and is therefore subject to saturation and competitive inhibition. Such mutual inhibition by two or more substrates is caused by the limited availability of sulphate and can be overcome by concomitant administration of sulphate or a sulphate donor³. Studies in man have demonstrated competitive inhibition of sulphate formation between acetaminophen and salicylamide, two widely used analgesics and antipyretics, and the prevention of this inhibition by concomitant administration of the sulphate donor L-cysteine⁴. We now report the results of a study which shows that concomitant administration of vitamin C and salicylamide causes a decrease in the conversion of the latter to salicylamide sulphate.