Spectrophotometry of carnitine in biological fluids and tissue with a Cobas Bio centrifugal analyzer

A spectrophotometric method for carnitine has been adapted to the Cobas Bio centrifugal analyzer. The addition of carnitine to a system containing carnitine acetyltransferase (EC 2.3.1.7) and acetyl-CoA gives rise to the formation of CoA. The system is coupled to 5,5'-dithiobis(2-nitrobenzoate) (DTNB). Assay response varied linearly with concentration of carnitine over a wide concentration range. The total CV was 5.5% for a carnitine concentration in serum of 58.0 mumol/L. Analytical recovery of carnitine added to a serum sample was 93%. No interference was found in icteric, not grossly hemolyzed, lipemic, or uremic sera. Comparison with a radioenzymatic method showed that results correlated well (r greater than 0.965) but the present method gave values proportionally greater by 10 to 25% for samples of plasma, dialysis fluid, urine, and muscle tissue. Advantages over the original spectrophotometric assays involving DTNB include low reagent costs, rapidity, simplicity, and reproducibility. However, this modification is not as sensitive and probably not as specific as the radioenzymatic methods.