il\VOLVEMENT OF RETII\OIC ACID II{ - MSpace at the University of ...

etinal intermediates and directly metabolizes beta-carotene into a 15,15'-enediol before
it is cleaved to catechol and uitimately to cis-cis mucinoic acid (Napoli and Race 1988)'
The cis-cis mucinoic acid then converts to retinoic acid through an unknown mechanism.
The study performed by V/ang et al. (1991) has shown that the incubation of liver, lung,
kidney and fat homogenates from several animals with beta carotene has resulted in the
formation of retinoic acid without a change in retinal levels. This indicates that retinoic
acid synthesis from beta carotene is not achieved through the retinal intermediate (Wang
et al. 1991). Although the complex process of retinoic acid is currently widely explored,
the exact sights of synthesis and enzymes involved in this process still remain to be
discovered.
IV.b.2.Retinoic acid transport: Retinoic acid is transported in plasma bound to albumin
(Blaner WS and Olson JA 1994). The fasting plasma levels of retinoic acid are generally
very low (1-14 nmoVl-) and represent 0.2-0.7 o/o of all of retinol plasma levels (Arnold et
aI. 1996; De Leenheer et al. 1982; Eckhoff and Nau 1990). The concentration of retinoic
acid in plasma of animals such as rat was found to be even lower l-7 nmoVL (Cullum
andZlle 1985; Napoli et al. 1985). The concentration of retinoic acid inplasma canbe
significantly altered by a dietary intake of vitamin A and its precursors. However, the
high fractional catabolic rate of retinoic acid (30.4 plasma pools/hr) indicates that plasma
levels of retinoic acid may be strictly maintained, despite an increased intake (Eckhoff et
al. 1991). It is thought that the most of retinoic acid is derived from retinol which is taken
by ,cells through the cellular membrane. The role of plasma RBP is still debatable,
however the current hypothesis is that there is a possibility of an existence of specific
RBP receptors on cellular membrane that facilitate trans-membrane transport of retinol
38