Serum Alkaline Phosphatase Isoenzymes in Lymphoproliferative Diseases1

A new isoenzyme of alkaline phosphatase (EC 3.1.3.1) has been reported to occur in sera from patients with lymphoproliferative diseases. This enzyme is character ized by an inability to hydrolyze cysteamine S-phosphate. We find that the 5,5 -dithobis(2-nitrobenzoic acid)-coupled assay method for cysteamine S-phosphate hydrolysis is not suitable for serum, and we were unable to confirm the existence of this isoenzyme in serum by this assay method. We were unable to detect this new isoenzyme by polyacrylamide gel electrophoresis with activity stains or by the reported high sensitivity of this enzyme to inhibition by cysteamine S-phosphate when p-nitrophenyl phos phate is the substrate. We were also unable to confirm reports of a unique inhibitor of normal alkaline phospha tase in the serum of patients with infectious mononucleosis.