Comparative structural and lectin-binding studies on γ-glutamyltransferase from human adult liver, fetal liver and primary hepatoma

Abstract

γ-Glutamyltransferase was purified to apparent homogeneity from human adult liver, fetal liver and hepatoma by deoxycholate extraction, immunoaffinity chromatography, papain digestion, phenyl-Sepharose chromatography and preparative polyacrylamide gel electrophoresis. The purified enzyme from all three sources had an apparent M_r of 82 000 by Sephadex G-150 gel filtration and on dodecyl sulphate/polyacrylamide gel electrophoresis two nonidentical subunits of M_r 57000 and 23000 were obtained. The pI of all three forms was 3.85 and after neuraminidase treatment they each gave at least five bands with pI values ranging over 5.9-6.6. Sialic acid content was 188 (adult liver), 182 (fetal liver) and 188 (hepatoma) nmol/mg protein. Total neutral sugar content was 702 (adult and fetal liver) and 700 (hepatoma) nmol/mg protein. The hexosamine content of the enzyme from all the three sources was the same (354 nmol/mg protein) and galactosamine was absent. Partially purified hydrophobic and hydrophilic forms of γ-glutamyltransferase from all the three sources were precipitated by Concanavalin A, Ricinus communis agglutinin and wheat germ agglutinin. These results show that γ-glutamyltransferase from human adult liver, fetal liver and hepatoma are structurally similar and that the elevated levels found in fetuses and hepatoma are only a quantitative increase and are not due to a new isoenzyme.