Die invloed van glikosilering van plasmafibrinogeen op die funksie en konsentrasie daarvan in diabetes mellituspasiënte

Abstract

Although it has been known for a number of years that early occlusive vascular disease has a higher incidence in the diabetic subject than in comparable nondiabetic subjects, the underlying reasons are unclear. Over the last decade several studies have been undertaken with a view to elucidate the underlying mechanism(s). Abnormalities in both lipid metabolism and haemostasis contribute to the development of vascular damage. In the early 80's Meade et al. first reported an association between haemostatic parameters and cardiovascular death. In 1984, Wilhelmsen et al, (1984) confirmed these observations and extended them to stroke. More recently, other prospective studies gave new impetus to . these observations and strengthened their clinical relevance. The abnormal haemostatic and fibrinolytic profiles of diabetic patients have been studied, but their contribution to occlusive vascular disease, and mechanisms of actions are still unclear. Therefore, advances in our knowledge of the basic mechanism(s) through which some abnormalities are involved in atherosclerosis and thrombosis are crucial to define the atherogenic potential of the abnormality and help to define appropriate strategies for intervention. Fibrinogen and its naturally occurring derivative, fibrin, are involved in mechanisms thought to play a key role in both atherogenesis and thrombosis. Glucose has been shown to bind non-enzymatically and through Schiff-base formation. irreversibly to fibrinogen This glycosylation appears to be dependant on the duration of exposure and glucose concentration in the surrounding medium. Because of this glycosylation, the tertiary or quaternary structure, function and/or degradation of the protein molecule may be altered, as has been demonstrated with glycated haemoglobin (HbA1) and many other proteins.

The aim of this study was to elucidate the effect of glycosylation of fibrinogen on the concentration and function of this molecule in diabetes mellitus subjects. Glycated proteins [glycated haemoglobin, (HbA1), fructosamine and glycated fibrinogen], proteins (fibrinogen, albumin and total protein), functional aspects of fibrinogen (clottability of fibrinogen, length-mass ratio and compaction of fibrin clots), lipoproteins [total cholesterol (TC), low density lipoprotein-cholesterol (LDLC), high density 1 ipoprotein-cholesterol ( HDL-C) and triglycerides (TG)] and fibrinolytic variables [tissueplasminogen activator antigen, tPA(Ag), and plasminogen activator inhibitor-1 activity, (PAI-1 activity)] of diabetes mellitus, non-diabetic hyperlipidaemic and control subjects were examined. Changes in these variables during a 16 week dietary intervention period in the diabetic subjects [high-fibre, high-carbohydrate, low-fat diet (HFHCLF) supplemented with either dry beans or a plant protein product] were also monitored. Serum lipids, glucose, albumin, total protein, fibrinogen, tPA(Ag), and PAI-1 activity were measured by means of colorimetric, enzymatic and immunological methods. Clottability and length-mass ratio were measured by means of turbidimetric methods. Compaction of fibrin clots was measured by means of a volumetric method and illustrated by means of electronmicrography. The results demonstrated significant increases in TC, LDL-C, TG, fibrinogen, PAI-1 activity, tPA(Ag), TP, HbA1, glucose and glycated fibrinogen in the diabetic subjects compared to control subjects. Length-mass ratio of fibrin, compaction of fibrin clots, albumin and HDL-C were significantly lower in the diabetic subjects. TC, LDLC, TG, fibrinogen, tPA(Ag), compaction of fibrin clots and length-mass ratio of fibrin in the hyperlipidaemic subject group also differed significantly from control subjects. A significant correlation between glycated fibrinogen and fibrinogen (r = 0.30 : p = 0.02) for the subject group as a

whole, was found. This may be an indication that glycation of fibrinogen plays a role in the enhancement of fibrinogen levels during hyperglycaemia. Fibrin clots in the diabetic subject group were characterized by long and thin fibrin fibers, which leads

to formation of rigid and compact fibrin clots. This type of fibrin clot structure has been shown to be less susceptible to degradation by plasmin. It also has a thrombogenic character (Blomback et al., 1992). The role of glycated fibrinogen in the type of fibrin clot formed, remains uncertain and should be investigated further. In the second part of the study, the correlation between the change of glycation of fibrinogen, induced by management of diabetes control by means of diet, and the function of fibrinogen, was measured in 16 diabetic subjects. An energy restricted HFHCLF diet, supplemented with 150 g dry beans (cooked weight) per day and the same diet supplemented with 50 g soy beans (raw weight) per day were prescribed to two paired/matched diabetic patient groups. The supplementation was sustained for 10 weeks. Improvement in diabetes management was shown by reduction of HbA1 in both subject groups. The changes between observed levels of glycated fibrinogen and functional aspects of fibrinogen could not be correlated. It is suspected that the main influence on the observed

functional properties of fibrinogen, was indirectly mediated through change in diabetes control. An independent effect of dry bean fibre on diabetes control, especially on PAI-1 activity was observed. In conclusion it seems that diabetes mellitus is associated with abnormal haemostatic function, characterized also by abnormal functional properties of the fibrinogen molecule, and that glycosylation of fibrinogen may contribute to this abnormality. An improvement in diabetes management is associated with a change in both functional aspects and glycation of fibrinogen. The underlying mechanism(s) are

however, still unclear.