In the present study, the kinetics of alkaline phosphatase (ALP) isoforms partially purified from rat liver, kidney, hypothalamus and cerebro-cortical region of brain is reported. The Michaelis-Menten constant (Km, micromoles) of the enzyme for the substrate p-nitrophenyl phosphate were 2.67, 2.50, 2.85 and 3.33 for the enzymes-obtained from liver, kidney, hypothalamus and cortical region respectively. The corresponding Vmax values (nanomoles/milligram protein/minute) were 1.03, 1.43, 1.33 and 2.22 respectively. The fraction of catalytic sites filled in the enzyme by the substrate was also calculated. For any substrate concentration, the fraction of catalytic sites filled was highest for renal ALP and lowest for cortical one. The variation in the properties of these tissue specific or unspecific enzymes may be due to differential expression of ALP genes or post-translational modifications. Since there was no significant difference between these values it was concluded that the kinetics of ALP for p-nitrophenyl phosphate substrate alone is not sufficient to distinguish these clinically important isoforms.
