The Conversion of P-hydroxybenzaldehyde to the Benzoquinone Ring of Ubiquinone in Rhodospirillum Rubrum.

The pathways involved in the biosynthesis of the benzoquinone ring of the ubiquinones have not been established. Recognition of the inability of mammals to synthesize aromatic rings de nova has led to the supposition that, in mammals, the benzoquinone ring of the ubiquinones arises from dietary aromatic amino acids. In support of this theory, Bentley et al. (1) administered phenylalanine-U-Cl4 to the rat and recovered 0.002 to 0.003% of the radioactivity in the quinone ring of liver and carcass coenzyme Q9.l They proposed that homogentisic acid was an intermediate in the conversion of phenylalanine to coenzyme Q9. However, Wiss, Gloor, and Weber (2) have concluded that tyrosine is not a precursor of coenzyme Q9 in the rat. In microorganisms, where several pathways for the synthesis of aromatic rings are available, no evidence has been reported that the aromatic amino acids serve as precursors of the ubiquinones. Thus, Rudney and Sugimura (3) found no measurable conversion of phenylalanine-U-Cl4 into coenzyme QB in yeast. Braun, Dewey, and Kidder (4) administered phenylalanine-U-C14 to Tetrahymena pyriformis, a microorganism auxotrophic for phenylalanine and tyrosine. On the basis of specific activity data, they rejected the possibility of direct conversion of the phenylalanine ring into the benzoquinone portion of ubiquinone. Some representative experiments described here explore the roles of several aromatic compounds in coenzyme Q10 biosynthesis in the photosynthetic microorganism Rlwdospirillum rubrum. Part of the incorporation of Cl4 into coenzyme Q10 when R. rubrw~m is grown with tyrosine-U-C14 has been traced to an impurity in the commercial tyrosine preparation. The impurity has been identified as p-hydroxybenzaldehyde. Evidence is offered that this substance is readily transformed into the benzoquinone ring of coenzyme Q1o and that, in this transformation, the aldehyde carbon is lost. If tyrosine gives rise to coenzyme Q10 by way of homogentisic acid, the /3 carbon of the tyrosine side chain might be expected to become the ring methyl group of coenzyme &lo (5). To test this, R. rubrum was grown in the presence of nn-tyrosine-3-Cl4 (1.63 pmoles, 4.0 X lo6 c.p.m. per pmole). The growth medium was that of Kohlmiller and Gest (6), and coenzyme Q10 was isolated (7) when the cell yields from 1 liter of medium were approximately 500 mg, dry weight. Although the cells took up about 40% of the radioactivity in these experiments, only 0.00013% of the Cl4 added appeared in coenzyme &lo (2.85