Three Immunologically Distinct Isozymes of Phenylalanine Hydroxylase

Three immunologically distinct and non-crossreacting isozymes of rat liver phenylalanine hydroxylase (PH) can be distinguished by immunotitration, immunodiffusion, and immunoabsorption. Only a single isozyme can be distinguished for rat kidney and minimal deviation hepatoma H4-II-E-C3 cell culture PH, by immunodiffusion and immunotitration. The ratios of antigen to antibody in milliunits of activity per milligram of immunoglobulin near equivalence by immunodiffusion of liver isozymes are 63, 21, 2.0, and 1.9 for kidney. The kidney (1.9 mU/mg of immunoglobulin) and the second liver (21 mU/mg of immunoglobulin) immunoprecipitin lines share immunologic identity as evidenced by their complete fusion. Immunoabsorption of the immunoglobulin with kidney PH with a ratio of antigen to antibody determined from near equivalence values obtained from immunotitration and immunodiffusion results abolishes the kidney and the second liver (21 mU/mg of immunoglobulin) immunoprecipitin lines. Additionally, retitration of this immunoabsorbed immunoglobulin abolishes all of the inhibition of kidney and shifts the initial immunotitration slope of liver PH to the left, but does not alter the second (2 mU/mg of immunoglobulin) slope. Immunotitration of H4-II-E-C3 minimal deviation hepatoma PH results in one slope with a near equivalence ratio of 2.1 mU/mg of immunoglobulin. No immunoprecipitin lines were observed on immunodiffusion for any of the dilutions of antigen or antibody tested. The behavior of H4-II-E-C3 PH is similar to antigen 3 of liver, and does not cross-react with the two other antibodies of liver. © 1975, American Chemical Society. All rights reserved.