A comparative study on the synthesis of the natriuretic hormone dopamine in OK and LLC-PK1 cells

The Vmax values (in nmol/mg protein/15 min) for AAAD in OK cells (0.94 +/- 0.08) were found to be significantly (P < 0.01) lower than those observed in LLC-PK1 cells (4.37 +/- 0.08). However, in both cell lines decarboxylation reaction was a saturable process with similar K(m) values (OK cells = 1.1 mM (0.3, 1.8); LLC-PK1 cells = 1.8 mM (1.6, 2.1)). Contrariwise to OK cells, decarboxylation of L-DOPA to dopamine in LLC-PK1 cells followed a linear (7.6 +/- 0.1 pmol/mg protein/min) non-saturable kinetics till 120 min of incubation. The formation of dopamine from increasing concentrations of L-DOPA (10 to 500 microM) followed a non-linear kinetics in both cell lines; the process of L-DOPA decarboxylation was saturated at low concentrations of L-DOPA with an apparent K(m) value of 11 microM (0.2, 22.6) in OK cells and 27.4 microM (11.1, 43.7) in LLC-PK1 cells. The formation of dopamine in LLC-PK1 cells (Vmax = 2097 +/- 113 pmol/mg protein/6 min) was 13.7-fold that occurred in OK cells (Vmax = 153 +/- 10 pmol/mg protein/6 min). In conclusion, LLC-PK1 cells appear to be endowed with a greater ability to form dopamine from exogenous L-DOPA when compared to OK cells.