The hybridoma technique was used to produce an allotypespecific monoclonal antibody (F11) that reacts with the products of the S, I, and D alleles of PLAP but not of the F allele. Serum and ascites samples from patients with different cancers containing high levels of PLAP were tested for reactivity with F11. These tumor-derived PLAPs were of the Nagao type as shown by their sensitivity to inhibition by l-leucine. This type of inhibition is exhibited also by the rare D allelic variant of PLAP but not by the common forms. Thus, it has been proposed that the Nagao enzyme represents reexpression of the D allele of PLAP. F11 reactive and nonreactive samples as well as samples with intermediate reactivity were found among the cancer sera and ascites. Our results show that the tumor-derived Nagao enzyme does not represent the reexpression of the D allele but instead, in spite of its distinct inhibition pattern, expresses the same genetic polymorphism that is found in the placenta.

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This work was supported by Grants CA 21967 and CA 27460 from the National Cancer Institute, Department of Health and Human Services and by Swedish Medical Research Council Grant 4217.

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