DECREASE OF RED-CELL MEMBRANE FLUIDITY AND -SH GROUPS DUE TO HYPERGLYCEMIC CONDITIONS IS COUNTERACTED BY ALPHA-LIPOIC ACID

Human red cell membranes (ghosts) were treated by 5 min of incubation with fasting or hypo- and hyperglycemic concentrations of D-glucose. T his simulation of nondiabetic or diabetic conditions revealed an influ ence on membrane fluidity and on protein -SH reactivity. Protein -SH g roups, measured with Ellman's reagent, generally behave in the same wa y as membrane fluidity determined with diphenylhexatriene. Maximal val ues were obtained with 5 mM D-glucose, whereas decrease was observed a bove 10 mM D-glucose. Addition of alpha-lipoic acid (4 nmol/mg protein ) resulted in a significant increase in membrane fluidity and titratab le -SH groups at glucose concentrations of 10 mM and above. Dithiothre itol diminished titrable -SH groups and did not restore membrane fluid ity. 2-Mercaptopropionylglycine was only effective in restoration of - SH groups. By contrast to D-glucose, other sugars such as L-glucose, D -fructose, or sucrose revealed no comparable changes on membrane fluid ity and titratable membrane -SH groups between concentrations of 5 and 10 mM. The hyperglycemic effects of D-glucose were corroborated with isolated, reconstituted membrane proteins and erythrocyte glucose carr ier, indicating that, in general, the observed divergent biochemical/b iophysical changes of the red cell membrane are influenced by the gluc ose transport protein GluT1. The natural R-form and the S-form of alph a-lipoic acid were compared with racemic R-/S-forms for their efficien cies in alterations of red cell membrane fluidity. Decreased fluiditie s in presence of 10 mM glucose were found to be influenced in differen tiated ways: the S-form was highly active in increasing fluidity at 4 nmol/mg and increasingly less active up to 20 nmol/mg protein. By cont rast the R-form of lipoic acid was moderately efficient in increasing fluidity through a larger concentration range between 4 and 80 nmol/mg protein. (C) 1995 Academic Press, Inc.