Isolation of a post-binding inhibitor of insulin action from the sera of non-insulin-dependent diabetics.

We have reported that sera from a majority of patients with non-insulin-dependent diabetes mellitus (NIDDM) inhibit insulin-stimulated lipogenesis (3-3H-glucose conversion to 3H-lipid) in rat adipocytes to a greater extent than control sera or sera from patients with insulin-dependent diabetes mellitus (IDDM). This effect was apparently due to a circulating, low molecular weight (Mr) substance soluble in acid/ethanol and resistant to proteases. We now describe the further characterization of this inhibitor isolated uniquely from NIDDM sera. Size exclusion chromatography of acid/ethanol extracts of sera on a Bio-Rad P2 column revealed the presence of a Mr 300-400 inhibitor of insulin-stimulated lipogenesis in 32 (70%) of 46 NIDDM sera but not in 9 IDDM or 12 control sera. NIDDM sera that contained the low Mr inhibitor had significantly elevated haemoglobin A1c levels compared with those without the inhibitor (14.1 +/- 2.6% vs 11.5 +/- 2.7%, p less than 0.001). In rat adipocytes, the low Mr inhibitor at a dilution equivalent to 1:40 unextracted serum, reduced maximal insulin-stimulated lipogenesis by 33%, 14C-6-glucose oxidation by 70% and 14C-1-glucose oxidation by 25%. In contrast, insulin-stimulated 2-deoxy-D-(1-3H) glucose uptake was unaffected. In the presence of the inhibitor, insulin sensitivity (insulin dose required for half-maximal response) was unchanged for lipogenesis but decreased for 14C-6-glucose oxidation and increased for 14C-1-glucose oxidation. In the presence of control sera, the low Mr inhibitor decreased basal lipogenesis (a measure of serum insulin-like activity) and its effect on insulin-stimulated lipogenesis was unaltered.(ABSTRACT TRUNCATED AT 250 WORDS)