Studies of Glutamate Dehydrogenase

Abstract

Cross-linking of the unimer of glutamate dehydrogenase from beef liver (consisting of six polypeptide chains each having a molecular weight of 56000) with dimethyladipimidate and subsequent analysis by sodium dodecylsulfate electrophoresis shows predominantely the trimeric species (molecular weight 168 000). Treatment with dimethylimidates of other chain length yields significantly less trimeric species indicating that the amino groups being cross-linked are within a distance of about 0.85 nm. Comparison of the molar amount of incorporated [14C]dimethyladipimidate with the number of modified amino groups (determined with trinitrobenzenesulfonic acid) shows that although 8 -9 of the 34 amino groups have reacted, only 2 -3 of them are involved in cross-links.Reaction with dimethylimidates inactivates the enzyme. The loss of the activity is partly concomitant to cross-linking to the trimeric species and not simply due to the modification of essential lysine residues. This is supported by the fact that, although more lysine residues react with monofunctional methylimidates, the loss of activity is reduced.Purified chymotryptic and tryptic peptides of the radioactive-labeled trimeric species were subjected to sequence analysis. Six peptides containing 75 % of the total label were identified: one involves the amino-terminal residue alanine-1 and the others involve lysine-105, lysine-I 54, lysine-269, lysine-358 and lysine-399. Quantitative analysis of the specific radioactivity of each peptide/mol lysine leads to the conclusion that only lysine-105, lysine-154, lysine-269 and lysine-358 participate in cross-links, lysine-269 and lysine-358, respectively, being at isologous and lysine-105 cross-linked with lysine-I 54 at heterologous contact domains of the enzyme.A model for the planar arrangement of the trimeric species in the quaternary structure of glutamate dehydrogenase is discussed. It includes both isologous and heterologous contact areas between the polypeptide chains.

A simplified route to [14C] dimethylsuberimidate for polypeptide crosslinking studies

“…In the study of proteins of known primary structure, the use of crosslinkingreagents may be extended to the isolation and identi~cation of the amino acid residues involved in crosslink formation and their location in the polypeptide chain [5,6]. The use of a radioactive reagent is clearly useful and often essential as a convenient means for monitoring crosslinked peptides during purification of these species from enzymic digests of crosslinked proteins.…”

The Use of Diazenedicarboxylic Acid Derivatives for Protein Cross-Linking. Use of Diazene for S-S Cross-Linking of Aldolase

“…Dimer, trimer and higher polymers are referred to as the associated unimers of aldolase of molecular weight 320000, 480000 and higher molecular weight, respectively. R-SH + R'-C-N = N-C-R' -+ R'-C-N-N-C-R' (1) where R represents a residue of a thiol compound and R' = N(CH2)4NCH3 with the N(CH2)4N in a piperazine ring. The second step of the reaction involves disulfide bridge formation with free -SH groups of the thiol compound [Eqn (2)].…”