A BSTRA CT Vitamin A circulates in human plasma as retinol bound to a specific transport protein. This protein differs from the known low and high density plasma lipoproteins and has a hydrated density greater than 1.21. In order to study this protein, volunteers were injected intravenously with retinol-15-14C. Plasma was collected 1-3 days later, and the purification of retinol-binding protein (RBP) was monitored by assaying for 14C and also by following the fluorescence of the proteinbound retinol. Purification of RBP was effected by the sequence: Cohn fractionation, chromatography on columns of Sephadex G-200 and diethylaminoethyl (DEAE)-Sephadex, preparative polyacrylamide gel electrophoresis, and finally chromatography on Sephadex G-100. These procedures resulted in a preparation of RBP which was at least 98%o pure and which had been purified more than 1500-fold. Purified RBP has al mobility on electrophoresis and has a molecular weight of approximately 21,000-22,000. There appears to be one binding site for retinol per molecule of RBP.

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Plasma transthyretin (TTR, formerly called prealbumin) is a 55-kd protein that participates in the plasma transport of both thyroxine and retinol (vitamin A). TTR concentrations are disproportionately high in human ventricular CSF, suggesting that TTR is either selectively transported across or synthesized de novo within the blood-CSF barrier. To address this question, we adopted a molecular genetic approach; after isolating a cDNA clone encoding human TTR, we previously demonstrated specific TTR messenger RNA (mRNA) synthesis in rat choroid plexus. We have now extended these investigations to the human brain. Northern analysis of postmortem brain homogenates revealed abundant TTR mRNA in choroid plexus, but not in cerebellum or cerebral cortex. Choroid plexus mRNA was readily translated into TTR preprotein in an in vitro translation system. An immunocytochemical survey of human postmortem brain sections revealed the presence of TTR protein specifically and uniquely in the cytoplasm of choroid plexus epithelial cells; these results were corroborated at the mRNA level by an extensive survey of whole rat-brain sections by in situ hybridization. Therefore, within the mammalian CNS, TTR is the first known protein synthesized solely by the choroid plexus, suggesting a special role for TTR in the brain or CSF. Whether this function differs from its established plasma transport functions is presently unknown.

ABS TR A CT The effects of diseases of the liver, the thyroid, and the kidneys on the retinol-binding protein (RBP)-prealbumin (PA) system responsible for the transport of vitamin A in plasma were examined, using a radial gel diffusion immunoassay for PA and the previously described radioimmunoassay for RBP. Measurements were made on plasma samples from 118 normal subjects, 31 patients with cirrhosis, 5 with chronic active hepatitis, 27 with acute viral hepatitis, 14 patients with hyperthyroidism, 7 with hypothyroidism, and 26 patients with chronic renal disease of varying etiologies. In the patients with liver disease, the levels of vitamin A, RBP, and PA were all markedly decreased and were highly significantly correlated over a wide range of concentrations. Serial samples were available in 19 patients with acute hepatitis; as the disease improved the plasma concentrations of vitamin A, RBP, and PA all increased.In patients with acute hepatitis RBP concentrations correlated negatively with the levels of plasma bilirubin, glutamic-oxaloacetic transaminase, and alkaline phosphatase. In the hyperthyroid patients both RBP and PA concentrations were significantly lower than normal; in hypothyroidism, neither protein showed levels significantly different from normal. In both hyper-and hypothyroidism and in liver disease, the molar ratios of RBP: PA and of RBP: vitamin A were not significantly different from normal.Patients with chronic renal disease had marked abnormalities in the plasma concentrations of RBP and vitamin A and in the molar ratios examined. In renal disease the levels of both RBP and vitamin A were greatly elevated, while the PA levels remained normal. The molar ratios of RBP: PA and of RBP: vitamin A were both markedly elevated. In many patients RBP was present in molar excess as compared with PA.

The plasma retinol transport system was studied in three patients with chronic hypervitaminosis A. The toxic state in each was associated with increased plasma concentrations of total vitamin A, and particularly of retinyl esters. The concentrations of plasma retinol-binding protein and prealbumin were, in contrast, non to retinol-binding protein. These limited clinical data support conclusions from detailed studies with hypervitaminotic rats, which suggest that vitamin A toxicity occurs when excessive amounts of vitamin A are presented to cell membranes in association with plasma lipoproteins, rather than specifically bound to retinol-binding protein. Retinol-binding protein may not only regulate the supply of retinol to tissues but also protect tissues from the surface-active properties of the vitamin.

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A B S T R A C T Cholesterol4-'C was injected intravenously into a series of normal men, untreated hyperlipidemic patients, and hyperlipidemic patients being treated with cholestyramine. The specific radioactivity of plasma total cholesterol was measured during the ensuing 10 wk. 16 studies were carried out in 10 subjects. Analysis of the turnover curves of plasma cholesterol revealed that in every study the turnover of plasma cholesterol conformed to a two-pool model. Each turnover curve was analyzed in terms of this model, as expressed by the equation: specific activity = CAe-at + CBeA0. The parameters which were calculated included the constants CA, CB, a, and 8; the size of the first pool (MA); the rate constants for the total rate of removal of cholesterol from each pool (kA and kBB); and the production rate in pool A (PRA). In two normal men and five untreated patients the average size of pool A-was 25 g.The effect of cholestyramine was assessed by comparing the results obtained without therapy with those obtained during therapy in five subjects studied under both conditions. Cholestyramine therapy produced a large increase in PRA (from 0.98 to 1.98 g/day) and in the rate of removal of cholesterol from pool A. Cholestyramine did not significantly alter the size of pool A.It is not possible to calculate the size of the total body exchangeable pool of cholesterol from the turnover curve of plasma cholesterol. It is also not possible to calculate the metabolic turnover rate,

The fact that vitamin A is absorbed via the lymphatic route was first demonstrated by Drummond, Bell, and Palmer in 1935, in studies carried out in a patient with chylothorax (2). Since then this finding has been confirmed and amplified by studies with other animal species. It has been well established that retinol 1 is largely esterified during its intestinal absorption, and partial information about the processes of absorption and esterification has been available from the studies of Ganguly and his associates (3-5) and Pollard and Bieri (6). In contrast to retinol, however, much less detailed information has been available concerning the intestinal absorption and metabolism of the provitamin A, ,8-carotene.Detailed studies of the events occurring during