Studies on Coenzyme Q

THE BIOSYNTHESIS OF COENZYME Q₉ IN RAT TISSUE SLICES

From the ¹ From the Department of Biochemistry and Nutrition, Graduate School of Public Health, University of Pittsburgh, Pittsburgh, Pennsylvania 15213

Unequivocal proof that individual rat tissues carry out the biosynthesis of coenzyme Q₉ (CoQ₉) from naturally occurring small molecules has been provided by the demonstration of incorporation of radioactivity from acetate-1-¹⁴C, DL-mevalonate-2-¹⁴C, uniformly labeled L-phenylalanine-¹⁴C, and uniformly labeled tyrosine-¹⁴C into CoQ₉ in rat liver, kidney, heart, and intestine slices. The rates of synthesis of CoQ₉ and cholesterol in various tissues were variable, although the rate of the over-all synthesis of cholesterol exceeded that of CoQ₉ by a ratio of 350:1 in rat liver slices and 15:1 in rat kidney slices.

The incorporation of mevalonate-2-¹⁴C into both cholesterol and CoQ₉ obeys saturation kinetics as measured by the ratio of specific activity of the product to precursor. The apparent K_m for cholesterol biosynthesis was 1.0 mM, whereas that for CoQ biosynthesis averaged 0.2 mM. Although the incorporation of label from mevalonate-2-¹⁴C into cholesterol was linear with time, a lag of about hour was observed before significant incorporation of label into CoQ₉ occurred. This lag in labeling of CoQ₉ was attributed to the accumulation of intermediates at a "slow" point in the reaction sequence.

The incorporation of mevalonate-2-¹⁴C into CoQ₉ in rat tissue slices was independent of the carbon dioxide tension in the reaction medium but was dependent upon the partial pressure of oxygen. Neither the addition of pyruvate as an exogenous energy source nor the additional rat serum stimulated the rate of biosynthesis in liver slices.

A radioactive fraction moving faster than CoQ₉ on reverse phase paper chromatography from the crude CoQ fraction isolated from alumina has been identified as a mixture of radioactive isoprenoid alcohols.

Tyrosine was approximately 3 times as effective as phenylalanine in labeling CoQ₉ in vitro and approximately twice as effective as acetate. Incorporation of label into CoQ₉ from the aromatic amino acids was little affected by addition of 5 mM nonradioactive mevalonate, whereas that from acetate-1-¹⁴C was inhibited 95% by means of isotope dilution. These findings support the dual origin of the side chain and aromatic portion of CoQ₉ and indicate the feasibility of studying its biosynthesis in vitro.

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