Urinary Excretion of Plasma Proteins in Diabetic Subjects: Increased Excretion of Kappa Light Chains in Diabetic Patients With and Without Proliferative Retinopathy

The urinary excretion of β₂-microglobulin, albumin, kappa light chains, transferrin, and IgG as well as their concentration ratios were assessed in 27 nondiabetic patients with proteinuria and in 72 IDDM patients, 41 with proliferative retinopathy (PR) and 31 without retinopathy, matched for age, duration of diabetes, and treatment. The mean excretions of albumin, transferrin, and IgG were similar in patients with nondiabetic proteinuria and in IDDM patients with PR and were significantly higher than in IDDM patients without retinopathy. Despite similar albumin excretion, the amount of excreted kappa light chains was significantly higher in IDDM patients than in patients with nondiabetic proteinuria, resulting in an elevated kappa chain/albumin ratio. Furthermore, diabetic subjects without microalbuminuria showed increased kappa chain/albumin ratio, indicating that increased urinary excretion of kappa chains may be an early sign of diabetic nephropathy. Determination of kappa light chain excretion may have clinical implications in the differentiation between proteinuria of diabetic and nondiabetic origin. The ratio kappa chain/albumin was independent of the excretion of β₂-microglobulin in patients with PR, suggesting that the reduced ability to reabsorb immunoglobulin light chains may occur earlier than that of β₂-microglobulin in the development of tubular dysfunction in insulin-dependent diabetes mellitus.