Discrimination of the isozymes of human placental hexosaminidase by kinetic parameter estimation

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Abstract
Human placental β-N-acetylhexosaminidase (EC 3.2.1.52) (HEX) is a lysosomal glycosyl hydrolase with an acidic pH optimum. Four isozymes (HEX B, HEX I1, HEX I2, and HEX A) have been isolated from human placenta. HEX BA derived from the subunit rearrangement of HEX A was also prepared. To determine if the isozymes of hexosaminidase differ in their kinetic parameters, the conditions for 4-methylumbelliferyl-β-D-N-acetylglucosaminide hydrolysis were optimized for each isozyme. The Km values were essentially similar and varied from 0.64 ± 0.06 for HEX BA to 0.85 ± 0.13 for HEX I1. The Vmax values were similar only for HEX I1 (3.90 ± 0.28 kat kg−1) and HEX I2 (4.40 ± 0.17). Vmax values varied significantly between HEX A (9.68 ± 0.52), HEX B (8.00 ± 0.75), HEX BA (4.81 ± 0.17), and the HEX I values.