Disparity between insulin and proinsulin on stimulation of hepatic tyrosine transaminase

The ability of insulin and proinsulin to stimulate tyrosine transaminase in dexamethasone-treated cultured rat liver cells was compared. Insulin increased this enzyme whereas proinsulin was seemingly without effect. Since proinsulin was not degraded by these cells, failure of stimulation of hepatic tyrosine transaminase could not be due to rapid destruction of the prohormone. This is the first demonstration of an action of insulin that cannot be duplicated by proinsulin.