Induction and ‘Superinduction’ of Sialylation of Membrane‐Bound γ‐Glutamyltransferase during Liver Regeneration

Abstract

The present paper shows that in the regenerating rat liver the membrane‐bound γ‐glutamyltransferase exists in two molecular forms. Depending on the state of proliferation, a sialic‐acid‐rich enzyme (in the fetal or regenerating liver) or a sialic‐acid‐poor enzyme (in the adult or quiescent liver) could be detected. In regenerating liver (24 h after 2/3 resection) only the sialic‐acid‐rich or fetal enzyme could be found. Since total enzyme activity (adult + fetal type) remained unchanged, it is assumed that the adult type of γ‐glutamyltransferase was modified by sialylation during the initial phase of liver regeneration. This process of sialylation was prevented by inhibitors of RNA or protein synthesis such as D‐galactosamine, actinomycin D or cycloheximide, provided that the inhibitor (D‐galactosamine) was given within the first 8 h after partial hepatectomy. Sialylation was not impaired by inhibitors of DNA synthesis, e.g. hydroxyurea or cytosine arabinoside. Administration of actinomycin D during a defined phase of proliferation (24 to 48 h after partial hepatectomy) stimulated the transfer of sialic acid to γ‐glutamyltransferase, a finding which describes for the first time the so‐called ‘superinduction’ of a sialylation process.