Metabolism of peripheral nerve myelin in experimental diabetes.

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Abstract
Previous in vitro studies of the metabolism of the peripheral nerve have been based on incorporation of radioactive precursor into components isolated from whole nerve. In this study we have determined incorporation secifically into myelin components of peripheral nerve by isolating myelin after incubating whole nerves with lipid or protein precursors and by determining the specific activity of the components of that membrane. The effect of diabetes on such incorporation was also studied. In the rat, in vitro incorporation of DL-[1-14C]leucine into protein components of myelin was decreased by 30-88% in diabetic animals as compared to controls. The major polypeptide constituent of rat sciatic nerve myelin (mol st 28,000; 58.5% of total mass of proteins) was not labeled in either the diabetic or the control group. In diabetes incorporation rate into a polypeptide of mol wt 23,000, which constitutes 21% of total mass, was approximately one half that of controls. In polypeptides of mol wt 38,000-49,000, which are heavily labeled in normal animals, but constitute only about 5% of total mass of proteins, depression of incorporation was e-en more marked in the diabetics. While these marked differences in incorporation between diabetic and control animals were observed, the amount of protein and its distribution among the constituent polypeptides was the same in both groups. In young rats made diabetic with streptozotocin and young rabbits made diabetic with alloxan, there was a lower rate of incorporation of the lipid precursors, [1-14C]sodium acetate or [3H]water, into myelin components. In older animals of both species incorporation in the controls was considerably lower than in the yount animals, and the effect of diabetes was no longer apparent. In nondiabetic animals, the in vitro addition of insulin (10-7 M) stimulated incorporation of DL-[1-14C]leucine into myelin proteins 1.6-3.1 times that of controls. This stimulation by insulin in vitro was not seen in diabetic animals. In animals in which diabetes had spontaneously recovered, however, incorporation rate in the in vitro experiments approached that of controls and were significantly above that in animals whose diabetes persisted. Since myelin is the palsma membrane of the Schwann cell, these studies provide evidence that the Schwann cell is affected by insulin and that some aspects of the metabolism of myelin are altered in insulin-deficient states.