Genetic hepatic lipase (HL) deficiency is associated with low density lipoprotein (LDL) rich in triglycerides (TG), whose affinity for B:E receptors is decreased. In rats, experimental hypoinsulinemia produces HL deficiency. However, the relation between human insulin-dependent Diabetes Mellitus (IDDM), HL activity and the characteristics of LDL have not been studied. The objective of our study is to evaluate the relation between HL activity and the chemical composition of LDL in treated IDDM patients. Subjects were 15 IDDM patients and 15 controls (C), matched for sex and body mass index (BMI). The IDDM patients were classified by the WHO criteria, were free of nephropathy and hypothyroidism, and received no medication except insulin. Controls were clinically healthy and normolipidemic with no family history of diabetes. The IDDM group was divided into two subgroups: subgroup IDDM-A (n = 9) with HL values > or = 4.3 and IDDM-B (n = 6) with HL < or = than 4.2 mumoles glycerol/ml h. the HL in IDDM was lower than in C (p < 0.001). Table 1 shows clinical data. Blood samples were drawn after 12 h fasting. Percentage of HbA1c and plasma concentrations of glucose, total cholesterol, LDL-cholesterol, HDL-cholesterol and TG were assayed. LDL was separated by sequential ultracentrifugation at densities of 1.019-1.063 g/ml and its chemical composition was analyzed. The most relevant results were: plasma TG concentration was higher in IDDM than in C (p < 0.05) (Table 2), although average values DMID not exceed the reference values of 200 mg/dl. The TG-LDL were higher in IDDM than in C: 24.8 +/- 2.7 vs 17.5 +/- 1.1 mg/dl plasma, media +/- SE, (p < 0.02). This difference reflected the values of IDDM-B, whose plasma concentrations of TG-LDL were higher than in C: 32.3 +/- 3.6 vs 17.5 +/- 1.1 mg/dl (p < 0.001), and also higher than in IDDM-A (p < 0.02). (Table 3). The chemical composition of LDL in IDDM-B contained a higher percentage of TG than C: 8.5 +/- 0.7 vs 6.8 +/- 0.3% (p < 0.05), a lower percentage of cholesterol than IDDM-A: 39.0 +/- 1.7 vs 45.2 +/- 2.2% (p < 0.05) and also a larger percentage of proteins than IDDM-A: 28.9 +/- 1.9 vs 20.8 +/- 1.0% (p < 0.01). The correlations between TG/cholesterol and HL activity in IDDM were r = -0.53 (p < 0.05) and in IDDM-B, r = -0.81 (p = 0.05). The noteworthy result of this study is the modification of the LDL particle in IDDM, rich in TG in patients with low HL activity. Anomalies in the chemical composition of LDL like those described decrease the uptake of this particle by its physiological B:E receptors. It has recently been demonstrated that LDL is an indisoluble association of lipids and apoproteins, and that both act simultaneously to hold the apoB in a spatial position that expresses normal epitopes. It has been described that particles of LDL rich in TG and poor in cholesterol, shows low affinity for LDL receptors in human fibroblasts. Also in IDDM the interaction of LDL rich in TG with B:E receptors is decreased. This might be one more mechanism contributing to the accelerated atherosclerosis of these patients. Our results suggest that there may be a threshold of HL activity for the complete hydrolysis of the TG of LDL, for the normalization of the TG/cholesterol relation and for the conformation of typical LDL particles.