Number of ways of joining SH groups to form multi-peptide chain proteins

The insulin A and B chains contain sufficient structural information to form the native molecule

Refolding studies show that native insulin can be reformed from A and B chains only. This suggests that the A and B chains contain the necessary structural information and that the C peptide is not required for this process.

The insulin A and B chains contain structural information for the formation of the native molecule. Studies with protein disulphide-isomerase

It has been shown previously [Tang, Wang & Tsou (1988) Biochem. J. 255, 451-455] that, under appropriate conditions, native insulin can be obtained from scrambled insulin or the S-sulphonates of the chains with a yield of 25-30%, together with reaction products containing the separated A and B chains. The native hormone is by far the predominant product among the isomers containing both chains. It is now shown that the presence of added C peptide has no appreciable effect on the yield of native insulin. At higher temperatures the content of the native hormone decreases whereas those of the separated chains increase, and in no case was scrambled insulin containing both chains the predominant product in the absence of denaturants. Both the scrambling and the unscrambling reactions give similar h.p.l.c. profiles for the products. Under similar conditions cross-linked insulin with native disulphide linkages can be obtained from the scrambled molecule or from the S-sulphonate derivative with yields of 50% and 75% respectively at 4 degrees C, and with a dilute solution of the hexa-S-sulphonate yields better than 90% can be obtained. The regenerated product is shown to have the native disulphide bridges by treatment with CNBr to give insulin and by the identity of the h.p.l.c. profile of its peptic hydrolysate with that for cross-linked insulin. It appears that the insulin A and B chains contain sufficient information for the formation of the native molecule and that the role of the connecting C peptide is to bring and to keep the two chains together.

Sulfhydryl proteins and the engram

A scheme, which is selectional not instructional, is described for the molecular basis of the engram. It uses as reactants intermediates formed during the catabolism of neuronal membrane sulfhydryl proteins. It is proposed that these compounds undergoing intermolecular association generate a large number of diverse structures, some of which are selected for further complex formation. The basis of selection is the binding of a particular compound to membrane lipid, which enhances its reaction with another bound compound. Because the composition and configuration of membrane lipids are influenced by environmental factors, the scheme implies that incoming sensory signals are matched with endogenously generated chemical structures.

Formation of native insulin from the scrambled molecule by protein disulphide-isomerase

The formation of native insulin either from scrambled insulin or from the separated A chain and B chain S-sulphonates by protein disulphide-isomerase was demonstrated with yields of 20-30% as measured by h.p.l.c. analysis, receptor binding and stimulation of lipogenesis. The h.p.l.c. profile of the reaction products shows that, among all the possible isomers containing both chains, the native hormone is by far the predominating product and consequently the most stable under certain conditions.

Resynthesis of insulin from its A and B chains in the presence of denaturants

Reoxidation of the reduced insulin A and B chains in 8 M urea leads to a recovery of native insulin of 2-7% whereas in 6 M guanidine or 0.5 mM dodecylsulfate very little, if any, resynthesis of insulin could be detected. Considering all the possibilities of different oligomeric forms containing one or both of the chains, the yield in 8 M urea is well over the yield as calculated from random joining of the chains and the yield in guanidine or dodecylsulfate is to be expected. It is concluded that some interaction and pairing of the chains occur even in the presence of 8 M urea.