Isolation, characterization, and spontaneous interconversion of two forms of human prostatic acid phosphatase

Starting with human prostatic acid phosphatase (HPAP) purified by affinity chromatography on Sepharose-bound N-(6-aminohexyl)tartramic acid [Van Etten, R. L. and Saini, M. S. (1978) Clin. Chem. 24, 1525], two forms of the enzyme can be separated by gradient elution DEAE-cellulose chromatography. The two enzyme forms are electrophoretically distinguishable on high pH gels. Under some conditions of storage one of these forms undergoes a conversion to the other. Both forms have identical amino acid compositions and Vmax values with typical substrates, but differences are observed in circular dichroism spectra, relative fluorescence intensities, Km values, and carbohydrate composition. It is concluded that the two forms differ only in the structure of their (nonphosphorylated) carbohydrate side chains. In view of the possible interconversion of the two forms upon storage, caution must be observed in any attempt to attach clinical significance to the relative amounts of the two forms.