522
P. F. Surai and I. I. Kochish
from various capsicum products varied from 1 to 6% (see Hamilton et al. 1990).
Therefore, only a small to moderate percentage of the carotenoid ingested with feed
will be taken up by the intestinal cells. Furthermore, in chickens, when the mucosal
cells are sloughed off into the lumen of the gastrointestinal tract, carotenoids which
were not incorporated into portomicrons are lost (Williams et al. 1998).
In poultry, lipoproteins synthesised in the intestinal wall are drained from the
intestines by portal blood. Thus, in the absence of a developed intestinal lymphatic
system in birds, absorbed lipids enter the portal system as large lipoproteins (called
portomicrons) with a diameter of about 150 nm (Annison 1983, Walzem 1996). They
consist of triglycerides, free and esterified cholesterol, phospholipids and apoproteins, and fat-soluble vitamins including carotenoids. Very little is known about the
cellular events that regulate or mediate the incorporation of carotenoids into portomicrons. This process could be facilitated by cytosolic binding proteins, but these have
not been reported in the intestinal mucosa. Thus, carotenoids together with other
lipids are transported as portomicrons directly via the portal system to the liver prior
to entry into the general circulation. In contrast, in mammals, chylomicrons are delivered into the plasma via the lymphatics and circulate through extrahepatic tissues
before passing through the liver. In the capillaries of certain extrahepatic tissues,
chylomicrons are hydrolysed by lipoprotein lipase (LPL), with subsequent transfer
of some carotenoids to the peripheral tissues and to other lipoproteins. In birds,
portomicrons partly undergo some transformation in the liver, but, probably, they
are largely released into the circulation unchanged or with minor changes. During
their transport in the circulation, portomicrons also undergo catabolism by LPL.
This enzyme is synthesised in several chicken tissues, but the largest amounts are
associated with skeletal muscle and adipose tissue (Stevens 1996). LPL is secreted
into the capillaries where it is bound on the luminal side of the endothelial cells.
Lipoproteins are substrates for endothelial bound LPL which catalyses intravascular triglyceride hydrolysis. During portomicron catabolism by LPL, the size of the
portomicron triglyceride core is reduced and excess surface is created. This excess
surface is transferred to HDL (Traber 1996) and a portion of the carotenoid content
is also transferred to HDL. Because HDL readily transfers vitamin E and probably
carotenoids to other lipoproteins (Traber et al. 1992), carotenoids are distributed
between all of the circulating lipoproteins. Portomicron remnants are cleared from
circulation by the liver and in this way carotenoids are delivered to this organ. The
factors controlling carotenoid uptake by various avian tissues and their transfer to
egg yolk are not known and this subject warrants further investigation. For example,
androgens modulate hepatic lycopene metabolism (Boileau et al. 2000). Although
a lot of different xanthophylls with biological properties are found in nature, only a
few are of industrial importance (mainly astaxanthin, canthaxanthin and lutein).
P. F. Surai and I. I. Kochish
from various capsicum products varied from 1 to 6% (see Hamilton et al. 1990).
Therefore, only a small to moderate percentage of the carotenoid ingested with feed
will be taken up by the intestinal cells. Furthermore, in chickens, when the mucosal
cells are sloughed off into the lumen of the gastrointestinal tract, carotenoids which
were not incorporated into portomicrons are lost (Williams et al. 1998).
In poultry, lipoproteins synthesised in the intestinal wall are drained from the
intestines by portal blood. Thus, in the absence of a developed intestinal lymphatic
system in birds, absorbed lipids enter the portal system as large lipoproteins (called
portomicrons) with a diameter of about 150 nm (Annison 1983, Walzem 1996). They
consist of triglycerides, free and esterified cholesterol, phospholipids and apoproteins, and fat-soluble vitamins including carotenoids. Very little is known about the
cellular events that regulate or mediate the incorporation of carotenoids into portomicrons. This process could be facilitated by cytosolic binding proteins, but these have
not been reported in the intestinal mucosa. Thus, carotenoids together with other
lipids are transported as portomicrons directly via the portal system to the liver prior
to entry into the general circulation. In contrast, in mammals, chylomicrons are delivered into the plasma via the lymphatics and circulate through extrahepatic tissues
before passing through the liver. In the capillaries of certain extrahepatic tissues,
chylomicrons are hydrolysed by lipoprotein lipase (LPL), with subsequent transfer
of some carotenoids to the peripheral tissues and to other lipoproteins. In birds,
portomicrons partly undergo some transformation in the liver, but, probably, they
are largely released into the circulation unchanged or with minor changes. During
their transport in the circulation, portomicrons also undergo catabolism by LPL.
This enzyme is synthesised in several chicken tissues, but the largest amounts are
associated with skeletal muscle and adipose tissue (Stevens 1996). LPL is secreted
into the capillaries where it is bound on the luminal side of the endothelial cells.
Lipoproteins are substrates for endothelial bound LPL which catalyses intravascular triglyceride hydrolysis. During portomicron catabolism by LPL, the size of the
portomicron triglyceride core is reduced and excess surface is created. This excess
surface is transferred to HDL (Traber 1996) and a portion of the carotenoid content
is also transferred to HDL. Because HDL readily transfers vitamin E and probably
carotenoids to other lipoproteins (Traber et al. 1992), carotenoids are distributed
between all of the circulating lipoproteins. Portomicron remnants are cleared from
circulation by the liver and in this way carotenoids are delivered to this organ. The
factors controlling carotenoid uptake by various avian tissues and their transfer to
egg yolk are not known and this subject warrants further investigation. For example,
androgens modulate hepatic lycopene metabolism (Boileau et al. 2000). Although
a lot of different xanthophylls with biological properties are found in nature, only a
few are of industrial importance (mainly astaxanthin, canthaxanthin and lutein).