Metabolic events in the intestine

Esterifi cation of retinol
Within enterocytes retinol becomes bound in a 1:1
molar ratio to CRBP-II, which is present exclusively
and abundantly in these cells. CRBP-II binds all-trans
and 13-cis retinol with high affi nity; it also binds retinaldehyde,
but not retinoic acid. The protein-bound
retinol is esterifi ed with saturated long-chain fatty
acids, preferentially palmitic acid. The esterifi cation
uses a different pool of fatty acids and hence
different enzymes than are used for the synthesis of
triglycerides. Two microsomal enzymes are involved
in the esterifi cation of retinol, namely acyl coenzyme
A:retinol acyltransferase (ARAT) and lecithin:retinol
acyltransferase (LRAT). The substrates for the ARATcatalysed
reaction are free retinol and acyl-CoAs;
retinol bound to CRBP-II is not a substrate for ARAT.
LRAT is an unusual enzyme in that it utilizes a membrane
phospholipid, phosphatidylcholine (lecithin),
as an endogenous donor of fatty acids for esterifi cation.
The enzyme activity shows positional selectivity
as only the fatty acid from position 1 of the phospholipid
is transferred to retinol. Position 1 is usually
occupied by a saturated fatty acid (palmitic or stearic
acid), which explains the predominance of saturated
fatty acids in the retinyl esters. Unlike ARAT, LRAT
can utilize CRBP-II-bound retinol as well as free retinol.
However, because of the abundance of CRBP-II in
enterocytes, the majority of retinol will be bound and
thus restricted to esterifi cation by LRAT.
Whether ARAT or LRAT is involved in retinol esterifi
cation depends on the amount of available retinol.
LRAT is responsible for the esterifi cation of
retinol–CRBP-II when normal loads of vitamin A
are ingested. In contrast, ARAT activity is important
if the amount of retinol absorbed exceeds the saturation
level of CRBP-II. It seems that CRBP-II functions
both to direct retinol to the microsomes for esterifi cation
by LRAT and to prevent retinol from participating
in the ARAT reaction.