Activity modulation of the fast and slow isozymes of human cytosolic low-molecular-weight acid phosphatase (ACP1) by purines

Abstract The activity modulation of homogeneous isozymes of the human cytosolic M r 18 000 acid phosphatase (ACP1) by purines has been investigated. A pronounced difference in the response of fast and slow isozymes of the same genetic type was observed, while identical properties were found for fast isozymes encoded by different alleles ( ACP1 × A, B and C ), as well as for the corresponding slow isozymes. The catalytic rate constant ( k c ) of the fast isozymes was increased 5.1-fold by hypoxanthine and decreased 40% by adenine, while the k c of the slow isozymes was unaffected by hypoxanthine but increased 4.6-fold by adenine. This finding and the genetically-determined differences in the relative quantities of the fast and slow isozymes account for the well-known phenotypic differences in activity modulation. The kinetic results strongly indicate that the effector binds to the free enzyme, as well as to the enzyme-substrate complex. Activating effectors showed a higher affinity for the free enzyme than for the enzyme-substrate complex, while the reverse was true with the inhibitor. The results exclude the possibility that effector and substrate bind to the same site of the enzyme; parasteric binding to adjacent sites is suggested.