Plasma total coenzyme Q9 (CoQ9) in the New Zealand population: reference interval and biological variation.

Coenzyme Q 9 (CoQ 9) in human plasma may originate as a product of incomplete CoQ 10 biosynthesis or from the diet. The estimated dietary CoQ 9 intake is 0 –1.3 ␮mol/day, primarily from cereals and fats (1, 2), but this is unreliable because many food items contain levels below the detection limit (1). Some methods for estimating human plasma CoQ 10 use CoQ 9 as an internal standard; thus, intraindi-vidual variation in the concentration of endogenous CoQ 9 is a potential source of error. We measured total CoQ 9 in human plasma, determined a reference interval , examined the biological variation , and documented the influence of CoQ 10 supplementation on plasma CoQ 9 concentrations. Self-reportedly healthy volunteers (n ϭ 193; men/women, 82/113) were enrolled for determination of a reference interval for CoQ 9. Mean age of participants was 45 years (range 18 – 82). No participants reported taking CoQ 10 supplements. Biological variation of CoQ 9 was determined in healthy adult male volunteers (n ϭ 10) who did not smoke and did not take vitamin or CoQ 10 supplements or medications in the 4 weeks before the study. Median age of the participants was 23.5 years (range 21–28). We took 7 blood samples from each participant in the morning after a 10-h overnight fast, at least 1 week apart, over a 2-month period. We determined the effect of CoQ 10 supplementation on plasma CoQ 9 concentrations using the same 10 individuals. After baseline blood samples were taken, we administered a CoQ 10 supplement (Q-Gel௡, Tishcon USA) as a single, nominal 150-mg dose. A vegetarian breakfast and lunch were provided. We obtained a 2nd blood sample 6 h after administration of the supplement. The plasma CoQ 9 and CoQ 10 assay , adapted from Tang et al. (3), measured total CoQ 9 and CoQ 10 simultaneously. We measured total cholesterol and direct LDL-cholesterol on all samples, using enzymatic methods (Aeroset, Abbott Laboratories), with coefficients of variation (CVs) of 1.6% and 1.2%, respectively. Blood samples were collected in lithium heparin, centrifuged within 1 h of collection, and plasma was stored protected from light: at Ϫ30 °C until analysis (maximum storage time, 112 days) for the reference interval study; and at Ϫ80 °C until analysis (maximum storage time, 150 days) for the biological variation and effect of CoQ 10 sup-plementation on plasma total CoQ 9 concentration studies. These studies were approved …