The fluorescence of serum proteins in diabetic patients with and without retinopathy

Non-enzymatic glycosylation of long-lived proteins results in a characteristic fluorescence, known as protein 'browning'. The degree of fluorescence of skin collagen correlates with retinopathy in diabetes. We have therefore measured the fluorescence of serum albumin and IgG by a sensitive HPLC technique in 69 diabetic patients, 38 with retinopathy and 31 without complications, and in 26 age-matched controls. The fluorescence of the IgG fraction, calculated as the ratio of fluorescence (excitation 360 nm, emission 454 nm) to optical density (280 nm), was elevated in diabetic patients with retinopathy (1.5 +/- 0.5) compared with those without retinopathy (1.1 +/- 0.2, p less than 0.005) and control subjects (1.1 +/- 0.2, p less than 0.005). The fluorescence ratio for the albumin fraction was increased in all diabetic patients (11.2 +/- 2.7) compared with control subjects (8.9 +/- 1.5, p less than 0.001). There was no significant difference in the fluorescence ratios of albumin in those with and without retinopathy. The fluorescence of serum albumin and IgG were not significantly correlated with serum protein glycosylation measured by the fructosamine method.