Intestinal variant alkaline phosphatase in plasma in disease

We investigated by enzyme electrophoresis after prolonged neuraminidase treatment the activity of "intestinal variant" (alpha 2-globulin mobility) alkaline phosphatase (EC 3.1.3.1; ALP) in the plasma of 189 patients selected for disorders (diabetes mellitus, liver cirrhosis, and chronic renal failure) with a known high frequency of increased plasma intestinal (beta-globulin mobility) ALP activity. The overall frequency of the variant ALP was 23.8%, whereas in the samples showing intestinal ALP it was 45.0%. The variant ALP was not observed in the absence of intestinal ALP, nor in patients of blood group A. Its frequency did not differ significantly between the different patient groups. Quantification of the variant ALP by densitometry was unsatisfactory but the quantity could be estimated by subtracting the intestinal ALP activity measured by electrophoresis from the activity determined by immunoassay with monoclonal antibody that reacts with both the intestinal and the variant forms. This indicated median activity of 12 U/L for the variant, approximately equal to that of the concomitant intestinal ALP. From the effects of papain and bromelain treatments, we suggest that "intestinal variant" represents intestinal ALP with attached membrane-binding domain.