A COMPARATIVE STUDY ON THE SYNTHESIS OF THE NATRIURETIC HORMONE DOPAMINE IN OK AND LLC‐PK 1 CELLS

The V max 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‐PK 1 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 m m (0.3, 1.8); LLC‐PK 1 cells=1.8 m m (1.6, 2.1)). Contrariwise to OK cells, decarboxylation of l ‐DOPA to dopamine in LLC‐PK 1 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 μ m ) 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 μ m (0.2, 22.6) in OK cells and 27.4 μ m (11.1, 43.7) in LLC‐PK 1 cells. The formation of dopamine in LLC‐PK 1 cells ( V max =2097±113 pmol/mg protein/6 min) was 13.7‐fold that occurred in OK cells ( V max =153±10 pmol/mg protein/6 min). In conclusion, LLC‐PK 1 cells appear to be endowed with a greater ability to form dopamine from exogenous l ‐DOPA when compared to OK cells.