Caco-2 cells in culture synthesize and degrade dopamine and 5-hydroxytryptamine: A comparison with rat jejunal epithelial cells

To explore the usefulness of Caco-2 cells in the study of intestinal dopami nergic and 5-hydroxytryptaminergic physiology, we have undertaken the study of aromatic L-amino acid decarboxylase (AADC), catechol-O-methyltransferas e (COMT) and type A and B monoamine oxidase (MAO-A and MAO-B) activities in these cells using specific substrates. The activity of these enzymes was a lso evaluated in isolated rat jejunal epithelial cells. The results showed that V-max values (in nmol mg protein(-1) h(-1)) for AADC, using L-DOPA as the substrate, in rat jejunal epithelial cells (127.3+/-11.4) were found to be 6-fold higher than in Caco-2 cells (22.5 +/- 2.6). However, K-m values in Caco-2 cells (1.24+/-0.37 mM) were similar to those observed in rat jeju anl epithelial cells (1.30+/-0.29 mM). Similar results were obtained when A ADC activity was evaluated using L-5HTP as substrate; in rat jejunal epithe lial cells V-max values (in nmol mg prot(-1) h(-1)) were found to be 5-fold that in Caco-2 cells (16.3+/-1.0 and 3.0+/-0.2, respectively), and K-m val ues in Caco-2 cells (0.23+/-0.08 mM) were again similar to those observed i n rat intestinal epithelial cells (0.09+/-0.03 mM). Caco-2 cells were not a ble to O-methylate dopamine, in contrast to rat jejunal epithelial cells (V -max = 8.6 +/- 0.4 nmol mg protein(-1) h(-1); K-m = 516+/-57 mu M). V-max v alues (in nmol mg protein(-1) h(-1)) for type A and B MAO in Caco-2 cells ( 19.0+/-0.6 and 5.4+/-0.6, respectively) were found to be significantly lowe r (P<0.05) than those in rat jejunal epithelial cells (46.9+/-3.1 and 9.6+/ -1.2, respectively); however, no significant differences in the K-m values were observed between Caco-2 and rat jejunal epithelial cells for both type A and B MAO. In conclusion, Caco-2 cells in culture are endowed with the s ynthetic and metabolic machinery needled to form and degrade DA and 5-HT, t hough, no COMT activity could be detected in these cells.