Distribution of prostatic acid phosphatase isoenzymes in normal and cancerous states

We have studied the IEF (isoelectric focusing) profiles and the sedimentation characteristics of intracellular and secretory prostatic acid phosphatase (PAP) in normal and cancerous states. IEF studies show a similar relative distribution of tartrate inhibitable pI 4.9 (approximately 80%) and 5.6 (approximately 20%) forms of this enzyme in normal as well as cancerous prostate. The same IEF profile is obtained regardless of whether an enzymatic or RIA method is utilized for detection of PAP. Of these two isoenzymes, only the form of pI 4.9 predominates in prostatic and seminal fluids and in Stage IV serum. Sedimentation analysis shows that the purified enzyme is exceptionally stable since it retains an S020,w value of 5.7 at low concentrations (ng/ml). While only the 5.7S form is observed in normal and cancerous tissues as well as in prostatic fluid, analysis of Stage IV serum reveals an additional form at 8.7S. Control experiments suggest that the 8.7S form is not induced by non-specific association with normal serum proteins or by the inhibitor tartrate. Our results suggest that: (a) of the two major isoenzymes in tissue, only the pI 4.9 isoenzyme predominates in secretion, (b) this relationship of intracellular to secretory forms is unaltered in the transition from normal to cancerous tissue, and (c) the utility of PAP as a tumor marker is derived at least in part by the intrinsic stability of the 5.7S form. The significance of the 8.7S form is unknown at the present time, but it does not distort the clinical (RIA) measurement of PAP in serum.