An increase in the carbohydrate moiety of α2-macroglobulin is associated with systemic lupus erythematosus (SLE)

Abstract

Using lectin blots in conjunction with peptide mapping, α2-macroglobulin micropurified from systemic lupus erythematosus (SLE) patients was shown to become abnormally glycosylated suggesting the occurrence of complex glycosylation in this pathological condition. To confirm there is indeed a quantitative increase in specific monosaccharides in this protein; α2-macroglobulin was micropurified from a battery of 37 serum samples which included 6 normal donors (3 male and 3 female), 23 SLE patients, 6 rheumatoid arthritis patients, 1 mixed connective tissue disease patient, and 1 Sjogren's syndrome patient; for carbohydrate analysis. It was noted that the concentration of total monosaccharides in α2-macroglobulin micropurified from serum samples of SLE patients is significantly higher than normal donors with a mean ± SD of 188 ± 410 μg/mg protein (SLE, n = 23) versus 14.5 ± 4 μg/mg protein (normal, n = 6) even though there was a high variation in the level of monosaccharides among the SLE patients. An increase in oligosaccharides in α2-macroglobulin from SLE patients compared to normal subjects was confirmed by concanavalin A (Con A) blots using peptide fragments derived from the micropurified protein. Since the interaction of peptide fragments derived from α2-macroglobulin with Con A requires the presence of mannose and/or glucose residues, we have also examined if there are any correlations between the levels of mannose and glucose in α2-macroglobulin and SLE. The concentration of mannose (38 ± 60 μg/mg protein) in α2-macroglobulin derived from SLE patients was significantly higher than normal donors (mannose, 4.8 ± 1 μg/mg protein) however, the concentration of glucose in α2-macroglobulin derived from SLE patients when compared to normal donors was not statistically significant, 18 ± 20 μg/mg protein in SLE versus 2 ± 0.5 μg/mg protein in normal donors due to high variation between samples. Also, the concentration of galactose in α2-macroglobulin from SLE patients was significantly higher than normal donors (45.7 ± 173 μg/mg protein versus 0.13 ± 0.03 μg/mg protein). These results illustrate quantification of carbohydrate in selected glycoproteins such as α2-macroglobulin may be a novel and alternative clinical marker for SLE.