Coupling of amines to and cross-linking of endogenous cytosol or membrane proteins by hepatic transglutaminase

Abstract

Endogenous protein acceptors for rabbit transglutaminase were demonstrated in the cytosol fraction of liver homogenates by the coupling of labeled compounds containing primary amino groups, such as putrescine, and by the apparent cross-linking of proteins, as indicated by a low mobility band observed in gel electrophoresis in 0.1% sodium dodecyl sulfate. When Ca 2+ was omitted or when iodoacetamide was added to either of these reactions, no coupling or cross-linking occurred. Cross-linking of proteins of isolated rabbit liver plasma membranes could be demonstrated in the presence of purified rabbit liver transglutaminase, Ca 2+, and a reducing agent. A marked diminution of at least two peaks corresponding to proteins of high molecular weight (> 150,000 daltons) was observed in densitometric scans of gels of proteins from membranes or cytosol cross-linked by the enzyme. It is proposed that these proteins participated in the crosslinking reaction. Since cross-linking was inhibited only weakly by relatively high concentrations of various amines, it is probable that a physiological role of hepatic transglutaminase is the catalysis of ϵ(γ-glutamyl) lysine bonds between cellular proteins, a process that might be controlled by the amount of intracellular Ca 2+ available to the enzyme.