Absorption, transport, and subcellular distribution of beta-carotene

Abstract

The search for possible in vivo functions of carotenoids has been complicated by the fact that answers to many basic questions concerning carotenoid absorption, transport, and subcellular distribution are not well defined.

In Chapter I, an in vivo brush border membrane vesicle (BBMV) model was developed for the study of intestinal uptake of carotenoids. The results indicated that BC was rapidly taken up into the BBMV membranes over a 5 minute period after which equilibrium was attained. This uptake was linearly related to BC micellar concentration up to the maximum level used in this study (6 uM), and is indicative of passive uptake.

In Chapter II, an assay was developed utilizing bovine liver and intestinal cytosol to study the possibility of protein-mediated BC transport between artificial membranes (liposomes) and natural membranes (mitochondria). The results of this experiment showed that BC transfer from liposomes to mitochondria did not increase upon addition of liver or intestinal cytosolic protein, and therefore BC does not appear to participate in protein-mediated transport in a manner similar to other lipid species.

In Chapter III, the subcellular distribution of all-trans and 9-cis BC in bovine liver was investigated. BC was found to be widely distributed among liver subfractions on a percent of total BC basis. The concentration of BC on a phospholipid basis was quite different between liver subfractions. The BC/PL ratios were highest for cream and cytosol, followed by nuclei, dense endoplasmic reticulum, light mitochondria, dense mitochondria, plasma membrane, and light endoplasmic reticulum. The concentration of BC was higher in the outer vs. inner mitochondrial membrane. 9-cis BC was distributed differently from all-trans BC, with the highest amount of total 9-cis BC present in the light mitochondrial fraction. The ratio of 9-cis/all-trans BC was also highest in the light mitochondrial and nuclear fractions.

In Chapter IV, a large carotenoid-lipid-protein complex ($>$2000 kD) was isolated from liver cytosol. SDS-PAGE analysis of this complex revealed the presence of 5 prominent protein bands (154, 126, 108, 92, and 58 kD) with numerous minor protein bands present.