Cutaneous xenobiotic metabolism: glycine conjugation in human and rat keratinocytes

Glycine conjugation is an important route of metabolism and detoxication of carboxylic acids in the liver. In this paper the in vitro cutaneous metabolism of [carboxyl-14C]benzoic acid to its glycine conjugate hippuric acid in rat and human skin is reported. Cutaneous glycine conjugation was studied in F344 rat and human epidermal keratinocytes using two systems: (1) freshly isolated keratinocytes in suspension and (2) primary keratinocyte cultures. For comparative purposes, studies were also carried out in freshly isolated and cultured F344 rat hepatocytes. After incubation of 5 × 106 cells with 1 μM benzoic acid at 37°C for 8 hr, no glycine conjugation was observed in rat and human keratinocyte suspensions, with greater than 98% of the radioactivity recovered as the parent compound. In contrast, cultured keratinocytes exhibited glycine conjugation, with 10.9 ± 1.0% (mean SEM, n = 3) and 2.1 ± 0.6% (mean SEM, n = 3) conversion to hippuric acid at 8 hr in rat and human cells, respectively. Tissue-specific differences in metabolism were observed, with conjugation in hepatocytes significantly greater (P < 0.05) than in keratinocytes at all times up to 8 hr. After incubation of benzoic acid with cultured hepatocytes for 8 hr, more than 98% of the of the radioactivity was recovered as the glycine conjugate. These studies indicate that rat and human skin possesses low, but demonstrable, glycine-conjugating activity, and that keratinocytes in primary culture may provide a better system than freshly isolated cell suspensions for studying such activity.