PURIFICATION AND PROPERTIES OF THE NATIVE FORM OF THE PURPLE ACID-PHOSPHATASE FROM BOVINE SPLEEN

The purple acid phosphatase (PAP) from bovine spleen has been shown to exist as a single ca. 36-kDa polypeptide in intact spleen tissue. The previously isolated microheterogeneous complex of 15-kDa and 23- or 2 1-kDa subunits appears to arise from proteolytic cleavage of an expose d, highly variable loop in the polypeptide chain. Small amounts of a s ingle polypeptide form, presumed to be the native form of the enzyme, have been obtained; this has permitted its optical and EPR spectra and fundamental kinetic properties to be determined. The most notable dif ference between the native and two-subunit forms of PAP is a ca. 3-fol d higher enzymatic activity for the latter, which is due to a simple i ncrease in V(max). The two forms are very similar spectroscopically an d chemically and appear to differ only in the loss of a highly antigen ic ca. five amino acid segment of the polypeptide between positions 15 5 and 160 but not in NH2-terminal sequence or in carbohydrate content. Analysis of published sequence data suggests that the existence of an exposed highly antigenic loop at positions corresponding to 155-161 o f the spleen PAP sequence is a relatively general feature of PAP's. Tr ypsin and chymotrypsin cleave both bovine spleen PAP and uteroferrin, apparently in this region, with significant enhancement of enzymatic a ctivity.