Hemoglobin Alc: Structure, Biosynthesis and Clinical Significance In Diabetes Mellitus
Diabetes mellitus is a chronic metabolic disorder characterized by abnormally high blood glucose concentrations and a relative or absolute deficiency of insulin. Approximately 5 per cent of the American population are afflicted with this disease, which is a leading cause of morbidity and mortality in the United States today. Diabetes may result in the dysfunction of many different organ systems, yet the biochemical mechanism(s) underlying these dysfunctions is unknown. Even the importance of hyperglycemia in the development of the sequelae of diabetes is unsettled. One of the metabolic abnormalities known to characterize this disease is an increase in the peripheral blood concentration of hemoglobin A1C. This minor red cell component comprises 3-5 per cent of the total hemoglobin in non-diabetic humans but up to 15 per cent in diabetics. There is evidence to suggest that it is a glycosylated derivative of hemoglobin A. The aim of these investigations has been to increase our understanding of the significance of increased hemoglobin A1C concentrations in diabetics. The studies described here in the diabetic mouse demonstrate increased hemoglobin A1C to be a marker for the diabetic phenotype regardless of the cause of diabetes. The increase in hemoglobin A1C concentration occurs 3-4 weeks after the onset of diabetes. Hemoglobin A1C is made as a post-synthetic modification of hemoglobin A at a constant slow rate throughout the life of the red cell. In diabetic humans, hemoglobin A1C concentration correlates with the severity of disease. Changes in the quality of diabetic control are followed, after a 3-4 week delay, by proportionate changes in hemoglobin A1C concentration. Hemoglobin A1C concentration reflects the mean blood glucose concentration for the 3-4 weeks prior to the measurement. Thus, infrequent hemoglobin A1C measurements would be sufficient to assess the quality of long-term diabetic control, a feature unique to this measurement. The periodic monitoring of hemoglobin A1C concentration should permit patients to achieve better diabetic control than is currently possible and should make it possible to determine whether hyperglycemia is important in the development of the sequelae of diabetes. The structure of hemoglobin A1C is identical to that of hemoglobin A, with the addition of l-deoxy fructose attached at the amino terminus of the B chains. The increased formation of hemoglobin A1C in diabetes is an example of the increased glycosylation of a protein (hemoglobin A) occurring in this disease process. The biosynthesis of hemoglobin A1C provides a conceptual framework that may explain the molecular basis for many of the sequelae of diabetes. Thus, the abnormal or excess glycosylation of other proteins may cause structural or functional changes in those proteins and thereby result in the diverse sequelae known to occur secondary to this disease.
diabetes, hemoglobin A1C, hyperglycemia, glycosylation, diabetic control, protein modification, Life Sciences
Networked Digital Library of Theses and Dissertations