Proteins of peripheral nerve myelin in diabetic neuropathy

A spin label study of myelin fluidity with normal and pathological peripheral nerves

The myelin fluidity in peripheral nerves from normal subjects and patients with neuropathy has been studied. Nerve biopsies were made in subjects without neuropathy and in patients with Guillain-Barré, alcoholic or diabetic polyneuropathies and heredodegenerative diseases. The fluidity was studied using a stearic acid spin label with a nitroxide radical at C12 (12-doxylstearic acid) which was introduced by exchange between spin label-charged BSA and nerve myelin. Spectra was obtained with a VARIAN E 109 ESR spectrometer, at temperatures from 5 to 45 degrees C. The ultrastructure of biopsied nerves was also studied. The electron spin resonance (ESR) spectra were all typical of a partially immobilized nitroxide with a distance between outer hyperfine lines (2T//) of about 50 G, at room temperature. The curve 2T// against temperature shows two critical temperatures: 10 and 30 degrees C for all nerves. No correlation could be established between ultrastructure and fluidity of nerves. However, the myelin of nerves from diabetic patients showed a significant increase in viscosity. This physicochemical modification may represent an initial alteration of the myelin sheath, and demands further investigation. No modification of the myelin fluidity was observed in any of the other nerves studied.

Studies on myelin proteins in human peripheral nerve

The myelin fraction from human peripheral nerve was prepared. Two basic protein fractions (BF-P2 and PB) were isolated from acid extracts of the myelin fraction and three glycoproteins (BR-PO, PASII and Y protein) were purified from its acid-insoluble residue. In biochemical analysis the human BF-P2 protein (M.W.13,000) showed similar but not identical properties to bovine BF-P2 protein. The PB fraction was suggested to include the encephalitogenic CNS-BP (M.W.18,000) and another, new protein of similar molecular weight. Both the human BR-PO protein (M.W.28,000) and PASII protein (M.W.13,000) showed similar biochemical properties to the corresponding myelin proteins of bovine peripheral nerve, while they both are clearly different from other myelin proteins. Close relationship between the BR-PO protein and the Y protein (M.W.22,000) was suggested by amino acid analysis. Injection of the myelin fraction of bovine peripheral nerve with the complete adjuvant produced EAN while the CNS-BP induced EAE in laboratory animals. However, all three purified proteins, BF-P2, BR-PO and PASII, from bovine peripheral nerve myelin were inactive in inducing demyelinating diseases.

Enzyme and protein studies of demyelination in diabetes

Neuropathological evidence of demyelination was found in the brain and sciatic nerve of diabetic patients at autopsy. The activity of acid proteinase was somewhat increased in the white matter but decreased in the gray matter of diabetic patients. No increase was observed in the activity of neutral proteinase in diabetic white and gray matter. The activities of beta-glucuronidase and 2',3'-cyclic nucleotide-3'-phosphohydrolase (CNP) were of the same level as those of the controls. The activities of all 4 enzymes appeared to be increased in the diabetic nerve, with the possible exception of CNP which was measured from only 1 nerve. Furthermore, the amount of total protein was markedly decreased in diabetic peripheral myelin. The encephalitogenic basic protein of diabetic brain myelin was normal in the disc gel electrophoretic patterns of brain myelin proteins. However, the basic proteins of peripheral myelin were reduced in a number of diabetic patients. The present biochemical findings for diabetic white and gray matter were largely normal. Instead, the increased activities of at least the proteinases and beta-glucuronidase in diabetic peripheral nerve, together with the loss of basic proteins, indicate extensive biochemical damage of the peripheral nervous system in diabetes. They suggest that demyelination and other phenomena observed in diabetic peripheral nerve are not caused only by angiopathy and impaired circulation.