Absorption, storage, and distribution of beta-carotene in normal and beta-carotene-fed rats: roles of parenchymal and stellate cells.

Absorption and storage of [14C]beta-carotene in control and beta-carotene-fed (BC-fed) rats were determined. Pre-feeding with beta-carotene for 2 weeks caused a 1.9-fold stimulation of its own absorption as well as its conversion to retinyl esters, whereas the absorption of [3H]retinyl acetate was unaffected. The liver and the lungs accounted for 60% and 30%, respectively, of the total recovered 14C radioactivity in both control and BC-fed groups. Beta-carotene accounted for 80-87% of the recovered 14C radioactivity in both the liver and the lung. Subcellular distribution of [14C]beta-carotene in both control and BC-fed groups revealed that the cytosol was the major fraction accounting for 44.4% and 26.8% of the radioactivity in the liver and lungs, respectively. Distribution of beta-carotene among liver parenchymal (PC) and stellate cells (STC) was determined in the two groups. Based on radioactivity, the PC and STC contained 22% and 78% of the total, respectively, in the control group; the corresponding values for the PC and STC in the BC-fed group were 48% and 52% of the total radioactivity, respectively. Based on the beta-carotene concentration following chronic beta-carotene feeding, PC contained 75.5% while the STC had 24.5% of the total beta-carotene. Thus, parenchymal cells seem to be the major hepatic storage site for dietary beta-carotene after chronic feeding.

A novel use of endpoint nephelometry to standardize the rate nephelometric assay of human and rat plasma apoprotein A.

A sensitive, accurate, and reliable method is described for calibrating the rate immunonephelometric assay of rat and human plasma apolipoprotein A (Apo A). Pure Apo A and high-density lipoprotein (HDL) of known Apo A concentration were used in endpoint nephelometry to determine Apo A concentrations of rat and human plasma pools. The endpoint method had coefficients of variation of 7.96% and 4.35% for rat and human plasma pools, respectively. These plasma pools were then used as secondary standards for the rate nephelometric assay. Excellent agreement (+/- 6%) existed between the plasma Apo A values determined by endpoint nephelometry and rate nephelometry. The Apo A concentration of a frozen human plasma pool determined by endpoint nephelometry was 125.2 +/- 9.6 mg/dl. The value of the same pool determined by rate nephelometry over a 1-year period with the Centers for Disease Control WHO lyophilized plasma standard was 125.4 +/- 21.2 mg/dl. Furthermore, it was found that the rat HDL was also a suitable standard in the rate nephelometric assay of Apo A. In contrast, Apo A, purified to homogeneity, showed different reaction kinetics from that of Apo A in the whole plasma and therefore was not a suitable standard in the rate nephelometric assay. We therefore conclude that primary standard Apo A, purified to homogeneity, can be used by endpoint nephelometry to calibrate plasma pools that can then be used as secondary standards in the rate nephelometric determination of rat and human plasma Apo A. The ready applicability of this method in the accurate determination of plasma Apo A under well-defined experimental conditions such as in chronic ethanol-fed rats and in human subjects with normal lipid levels and those with hyperlipidemia is demonstrated.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on lipid synthesis and lipogenic enzymes in the rat.

A single i.p. administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) caused within 1 wk of exposure a dose-dependent progressive inhibition of liver fatty acid synthetic rate with concomitant decreases in hepatic fatty acid synthetase and acetylcoenzyme A carboxylase activities. Similarly, hepatic cholesterol synthetic rate was markedly inhibited with increasing dosage of TCDD, although the corresponding decrease in 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity was of lesser magnitude. Linear regression analyses of the reciprocals of the responses versus the dose revealed that the TCDD concentration which caused 50% inhibition of the activities of various lipogenic enzymes and of lipid synthetic rates ranged from 11 to 20 micrograms/kg (34-67 nM) with an average of 15 micrograms/kg (47 nM). Hepatic cholesterol synthesis seemed to be more sensitive to inhibition than fatty acid synthesis whether it was based on TCDD dosage or duration of exposure. The degree of inhibition of all the above parameters except fatty acid synthesis in liver and adipose tissues increased from 1 to 2 wk of exposure but was less pronounced after 4 wk exposure. Significantly, the adipose tissue was found to be more sensitive than the liver with respect to inhibition of fatty acid synthesis by increasing dosage of TCDD. Thus, the biochemical mechanism of loss of adipose mass caused by TCDD exposure may well be mediated by strong inhibition of lipid synthesis in the adipose tissue coupled with increased mobilization of depot fat.