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

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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.