Quantification of physiological levels of retinoic acid.

The methods discussed here are versatile procedures that have been effective for the quantification of retinoic acid and retinol in plasma or serum, cells in culture, and animal tissues. They are capable of measuring a wide range of concentrations: the GC/MS assay for retinoic acid is especially useful when low amounts of retinoic acid are to be accurately quantified and is unrivaled for sensitivity and specificity; the HPLC assays are less sensitive, but are more convenient, more accessible, and preclude the need for the expensive and demanding technology of GC/MS. With these procedures, retinoic acid has been measured in human plasma (approximately 4.9 ng/ml), rat serum (approximately 2 ng/ml), fetal bovine serum (approximately 2 mg/ml), and rat tissues (0.05-1.0 microM). In the first two cases, all-trans-retinoic acid accounts for approximately 75% of the total. The proportion of all-trans-retinoic acid in other cases has not yet been determined.