The Sulfur of Insulin

Dudley (1) was probably the first to hint at the possibility that insulin contained sulfur, but he did not feel justified in drawing any conclusions as to the relation between activity and sulfur content.He marked the very strong organic sulfur reaction given by his purified product when boiled with lead acetate and sodium hydroxide.He was unable to obtain, however, a positive nitroprusside reaction on insulin itself or in a reduced solution of his "insulin hydrochloride."He was therefore convinced that the sulfur in the intact substance was not reducible as it is, for example, in glutathione.Piper, Allen, and Murlin (2) and Shonle and Waldo (3) also reported a negative nitroprusside test for the sulfhydryl group.These investigators made the interegting observation that after boiling with acid the leadblackening sulfur was reduced three-fourths.Blatherwick (4) likewise obtained a negative test on the intact insulin.Upon reduction, however, with sodium cyanide he was able to get a very distinct positive test, showing that although the sulfhydryl group, as such, is not r~ :sent inI the insulin material the sulfur is present in a reducible fc,m.It .remainedfor Abel and Geiling (5) to demonstrate the high degree of lability of the sulfur and its relationship to potency.They found that merely boiling with so weak an alkali as 0.1 N sodium carbonate so affected the sulfur that upon acidification hydrogen sulfide was given off.These authors felt justified from their work to draw the conclusion that the labile sulfur was directly proportional to the degree of hypoglycemic activity and that "this unstable sulfur is an integral part of the insulin moleculeand that the alteration in its condition consequent upon heating with sodium carbonate bears to the destruction of the physiological activity of the hormone the relation of cause to effect."Abel (6) has also found that his crystalline insulin contains sulfur in a labile form.In a recent publication (7) he and his collaborators have confirmed this finding although their method of obtaining the crystals is somewhat different.1 1 I have recently been able to isolate by the method devised by Abel ,ind his coworkers a small quantity of crystals identical with those de-393