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Two new symmetrical bis(imido esters), N,N'-bis(2-carboximidoethyl)tartarimide dimethyl ester dihydrochloride and N,N'-bis(2-carboximidomethyl)tartarimide dimethyl ester dihydrochloride, have been synthesized. Tests with the tetrameric enzyme, fructose diphosphate aldolase, show that these reagents closely resemble dimethyl suberimidate in their ability to cross-link protein subunits. However, identification of the cross-linked species, separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis, is greatly facilitated since the cross-links can be broken by a simple treatment with sodium periodate. The periodate cleavage step can be introduced between the two dimensions of a diagonal gel electrophoretic separation, the contributors to a cross-linked species then moving off the diagonal formed by uncross-linked proteins and reverting to the positions in the gel that correspond with their regenerated monomeric form. When the pyruvate dehydrogenase multienzyme complex of Escherichia coli was treated with dimethyl suberimidate or N,N'-bis(2-carboximidoethyl)tartarimide dimethyl ester dihydrochloride, cross-links rapidly formed between the subunits of the transacetylase and lipoamide dehydrogenase components. On the other hand, cross-links failed to form between the subunits of the decarboxylase component themselves, or between the decarboxylase and the other two types of subunit in the complex. Cross-linking experiments with the isolated lipoamide dehydrogenase were compatible with the accepted dimeric structure of this enzyme is free solution, whereas the isolated pyruvate decarboxylase component also failed to cross-link when treated with dimethyl suberimidate in free solution. The cross-linking experiments with the intact multienzyme complex provide evidence for the existence of the lipoamide dehydrogenase dimer in the assembled enzyme and show the need to interpret such experiments with care since, from other evidence, the pyruvate decarboxylase component is known to be bound to the transacetylase "core" of the complex.

The intramolecular passage of substrate between the component enzymes of the pyruvate dehydrogenase multienzyme complex of Escherichia coli was examined. A series of partly reassembled complexes, varying only in their E1 (pyruvate decarboxylase, EC 1.2.4.1) content, was incubated with pyruvate in the absence of CoA, conditions under which the lipoic acid residues covalently bound to the E2 (lipoate acetyltransferase, EC2.3.1.12) chains of the complex become reductively acetylated, and the reaction then ceases. The fraction of E2 chains thus acetylated was estimated by specific reaction of the thiol groups in the acetyl-lipoic acid moieties with N-ethyl[2,3-14C]maleimide. The simplest interpretation of the results was that a single E1 dimer is capable of catalysing the rapid acetylation of 8-12 E2 chains, in good agreement with the results of Bates, Danson, Hale, Hooper & Perham [(1977) Nature (London) 268, 313-316]. This novel functional connexion of active sites must be brought about by transacetylation reactions between lipoic acid residues of neighbouring E2 chains in the enzyme complex. There was also a slow transacylation process between the rapidly acetylated lipoic acid residues and those that did not react in the initial, faster phase. This interaction was not investigated in detail, since it is too slow to be of kinetic significance in the normal enzymic reaction.

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