Application of O-(4-diazo-3-[125I]iodobenzoyl)sucrose for the detection of the catabolic sites of low density lipoprotein

Effect of 17 alpha-ethinylestradiol on the catabolism of high-density lipoprotein apolipoprotein A-I in the rat

The in vivo metabolism and tissue sites of catabolism of high-density lipoproteins (HDL), labelled specifically in the apolipoprotein (apo) A-I moiety, were studied in rats treated with 17 alpha-ethinylestradiol (EE) for 5 days. Apo A-I was labelled either with O-(4-diazo-3-[125I]iodobenzoyl)sucrose, a non-degradable labelling compound, or with 131ICl. It was found that EE treatment decreases the serum cholesterol concentration to 10 mg/dl and stimulates the serum decay of apo A-I labelled HDL. The latter effect could be attributed to an increased catabolism of apo A-I labelled HDL in the liver. The increased rates of the serum decay and tissue uptake of apo A-I labelled HDL in EE-treated rats were not affected by a bolus injection of unlabelled human low-density lipoprotein (LDL), administered at the time of the injection of the labelled HDL. When the serum cholesterol concentration was raised to physiological levels by a bolus injection of unlabelled rat HDL, both the serum decay and the tissue uptake of apo A-I labelled HDL were almost completely restored to conditions encountered in control animals. In vitro binding experiments showed that liver membranes obtained from EE-treated rats demonstrated a 6-fold increased specific binding of human 125I-LDL, but virtually unchanged specific binding of rat 125I-HDL, as compared with liver membranes obtained from control rats. It is concluded that rat HDL apo A-I catabolism is hardly mediated by the apo B/E receptor induced by EE treatment.

The sites of degradation of purified rat low density lipoprotein and high density lipoprotein in the rat

Low density lipoprotein and high density lipoprotein were isolated from rat serum by sequential ultracentrifugation in the density intervals 1.025-1.050 g/ml and 1.125-1.21 g/ml, respectively. The isolated lipoproteins were radioiodinated using ICl. Low density lipoprotein was further purified by concanavalin A affinity chromatography and concentrated by ultracentrifugation. 95% of the purified low density lipoprotein radioactivity was precipitable by tetramethylurea, while only 4% was associated with lipids. The radioiodinated high density lipoprotein was incubated for 1 h at 4 degrees C with unlabelled very low density lipoprotein, followed by reisolation by sequential ultracentrifugation. Only 3% of the radioactivity was associated with lipids and 90% was present on apolipoprotein A-I. The serum decay curves of labelled and subsequently purified rat low and high density lipoprotein, measured over a period of 28 h, clearly exhibited more than one component, in contrast to the monoexponential decay curves of iodinated human low density lipoprotein. The decay curves were not affected by the methods used to purify the LDL and HDL preparations. The catabolic sites of the labelled rat lipoproteins were analyzed in vivo using leupeptin-treated rats. In vivo treatment of rats with leupeptin did not affect the rate of disappearance from serum of intravenously injected labelled rat low density lipoprotein and high density lipoprotein. Leupeptin-dependent accumulation of radioiodine occurred almost exclusively in the liver after intravenous injection of iodinated low density lipoprotein, while both the liver and the kidneys showed leupeptin-dependent accumulation of radioactivity after injection of iodinated high density lipoprotein.

In vivo catabolism of human low density lipoprotein in the rat is mediated by a nonsaturable, low-affinity mechanism

The degradation of human low density lipoprotein (LDL) was analyzed in fasted rats treated for 3 days with either 4-aminopyrazolo-(3,4-d)pyrimidine (4APP) or saline. Treatment with 4APP caused an 80% decrease in serum cholesterol concentration. The mono-exponential serum decay of a tracer amount of labelled LDL was changed neither by 4APP treatment, nor by the simultaneous injection of a bolus of unlabelled LDL. The sites of degradation of human LDL were determined using the nondegradable labelling compound O-(4-diazo-3-[125I]iodobenzoyl)sucrose (D125IBS). The sites of degradation and the rate of degradation of D125IBS labelled LDL were also not affected by 4APP treatment or by injection of a bolus of unlabelled LDL. It is concluded that human LDL is catabolised in the rat by way of a nonsaturable, low-affinity mechanism.