Vitamin A metabolism: analysis of steady-state neutral metabolites in rat tissues

High-performance liquid chromatography systems were developed to rapidly separate retinol from its esters, analyze the total spectrum of neutral vitamin A compounds, and purify retinyl esters to homogeneity. Chemical ionization mass spectrometric techniques were used to identify vitamin A compounds; these techniques are also applicable to quantification of tissue vitamin A compounds. These methods provide rapid and sensitive techniques for separation and quantification of neutral retinol metabolites. Their utility was demonstrated by analysis of vitamin A metabolites in rat tissues under steady-state conditions. Tissue specificity was noted for the concentrations of retinol and its long-chain fatty acid esters, the ratio of retinol to retinyl esters, and the fatty acid composition of retinyl esters. Quantitatively minor amounts of several neutral polar retinol metabolites were detected, but neither 13-cis-retinol nor 4-hydroxyretinol was observed in vivo as metabolites of retinol in kidney.