ABSTRACT
The mechanism by which retinol is recycled from tissues to plasma is of interest. Earlier it was suggested that retinol, presumably as retinyl esters, might recycle on plasma lipoproteins. Data from one of the first of the tracer kinetic experiments referred to above (5) indicated instead that the primary vehicle for the extensive retinol recycling is plasma retinol-binding protein (RBP), since <5% of total plasma radioactivity was in retinyl esters at all timeS after intfaVCilmi!§ administration Of eH]retin0l-labeled plasma. 4"M'HU<AJ9U recycling on was included in the model, this route could not account for the extent of observed. In recent years, it has been '""'"'"rt••n that many extrahepatic including kidney, eye, and adipo§e contain mRNA for RBP Thus although the source of RBP for
tissues is not yet known, it is vvo'""'''"' may lbe used for export of retinol from
hepatic tissues. In addition to questions about the vehicle responsible for retinol recycling from
tissues, it is also not yet known in what form retinol moves from plasma to the interstitial space to cells. It is well accepted that essentially all of the retinol in plasma is carried by the retinol-binding protein-tmnsthyretin (RBP-TIR) complex, but how does retinol move to cells? The following considerations indicate that the vast majority of retinol probably leaves the circulation as RBP-retinol. Based on the work of Fex and Felding and and Blaner (14), it can be estimated that ~95.5% of plasma retinol is present as TI'R-RBP-retinol, -4.4% as REP-retinol, and ~0.14% as unbound retinol. Using these distributions and in consideration of molecular for retinol, 21,286 for REP-retinol, and 76,266 for and capillary permeabilities, it seems that most of the retinol leaves the circulation as REP-retinoL Because TIR-RBP-retinol would leave the circulation too slowly [transit time :> 10 hr based on albumin kinetics (15)] and there is too little unbound retinol of a high filtration fraction into to account for the high rates of plasma retinol turnover observed in kinetic studies (5,6,8). This conclusion was also drawn by Fex and Felding (13).
