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

Abstract

The V maxvalues (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 1cells (4.37±0.08). However, in both cell lines decarboxylation reaction was a saturable process with similar K mvalues (OK cells=1.1 m M(0.3, 1.8); LLC-PK 1cells=1.8 m M(1.6, 2.1)). Contrariwise to OK cells, decarboxylation of L-DOPA to dopamine in LLC-PK 1cells 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 mvalue of 11 μ M(0.2, 22.6) in OK cells and 27.4 μ M(11.1, 43.7) in LLC-PK 1cells. The formation of dopamine in LLC-PK 1cells ( 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 1cells appear to be endowed with a greater ability to form dopamine from exogenous L-DOPA when compared to OK cells.