Biological Dehydroxylation

Abstract

THE hydroxylation of aromatic compounds in the animal body to form phenols is a well-established biochemical reaction (see review¹) and has been known for nearly a hundred years. It was in 1867 that Schultzen and Naunyn² first showed that benzene was converted to phenol in man and the dog. The reverse reaction, namely, dehydroxylation, is not well known. The first suggestion that such a reaction occurred in vivo was published in 1955 when Booth, Murray, DeEds and Jones³ noted an enhancement of the m-hydroxyphenylacetic acid output in the urine of rabbits that had been dosed with 3,4-di-hydroxyphenylacetic acid (homoprotocatechuic acid) or with the flavonoids, rutin and quercetin. It was suggested that the m-hydroxyphenylacetic acid arose by loss of the para-hydroxyl group of 3,4-dihydroxy-phenylacetic acid. The excretion of increased amounts of m-hydroxyphenyl acids has since been observed after the administration of 3,4-dihydroxycinnamic acid (caffeic acid) to man, the rabbit and the rat^4,5 and after 3,4-dihydroxyphenylalanine (dopa) to the rabbit and the rat⁴. The ingestion of coffee leads to the excretion of increased amounts of m-hydroxyhippuric acid in human urine⁶, and this has been ascribed by Booth et al. ⁵ to the breakdown of chlorogenic acid, which is the quinic acid ester of caffeic acid, and which occurs in coffee beans to the extent of 5 per cent on a dry weight basis. According to Armstrong et al. ^6,7, m-hydroxybenzoic, -phenylacetic, -phenylpropionic and probably -phenyllactic acids occur in human urine, but the origin of these meta-acids is obscure. However these acids could arise during metabolism by dehydroxylation of 3,4-dihydroxyphenyl acids (catechol acids). It is thus possible that dehydroxylation is a reaction of considerable metabolic significance.