the episodic nature of digesta entry in meal-
feeding non-ruminants. Recent evidence
also indicates that some triacylglycerol-
rich lipoproteins may be secreted into the
portal vein of calves and functioning
ruminants (Bauchart, 1993).
Fatty acid digestibility in ruminants is
usually lower and more variable than that
in non-ruminants (Doreau and Chilliard,
1997). Intestinal digestibility does not differ
appreciably between 16- and 18-carbon
fatty acids (average of 79 and 77%, respec-
tively), and is slightly greater for
unsaturated than saturated fatty acids (77,
85 and 83% for 18:0, 18:1 and 18:2, respec-
tively; Doreau and Chilliard, 1997).
Lipid transport: lipoprotein metabolismWith the exception of free fatty acids,
which circulate bound to serum albumin,
lipids circulate as components of large
lipoprotein particles. Lipoproteins generally
are classified according to their buoyant
density, which is determined by the rela-
tive proportions of lipids and proteins. The
largest lipoproteins are the chylomicrons,
followed by VLDLs. These are also the least
dense materials because they carry large
lipid loads with relatively small protein
contents. High-density lipoproteins (HDLs)
are the smallest particles and the most
dense, having higher amounts of protein
and less lipid. Low-density lipoproteins
(LDLs) have densities between those of
HDLs and VLDLs.
Intestinally derived lipoproteins rich in
triacylglycerols (chylomicrons, VLDL) func-
tion to deliver dietary long-chain fatty acids
to peripheral tissues (Fig. 5.2). The liver
also secretes VLDLs as a way to package
endogenous triacylglycerols for transport in
plasma. Following secretion from intestinal
cells or liver, these triacylglycerol-rich
lipoproteins acquire apo-CII from circulat-
ing HDLs (Hussain et al., 1996). Apo-CII is
an activator of the enzyme lipoprotein
lipase (LPL), which is responsible for clear-
ance of plasma triacylglycerol (Braun and
Severson, 1992). LPL is present in most
tissues and is found in high activities in
adipose tissue, lactating mammary gland,
heart and skeletal muscle. Synthesis of LPL
occurs in the parenchymal cells of the
tissue; the LPL is secreted from the cells
and translocated to the interior surfaces of
capillaries perfusing the tissue. There, the
highly glycosylated LPL is anchored to the
vascular surface of the endothelial cells by
interactions with heparin sulphate proteo-
glycans on the cell surface (Braun and
Severson, 1992).
As chylomicrons and VLDLs move
through the capillary beds, they become
trapped by LPL through interactions of the
carbohydrate moieties of apo-B and LPL.
Binding is facilitated by the presence of
apo-CII in the triacylglycerol-rich lipo-
proteins. Triacylglycerol hydrolysis occurs
rapidly, with release of free fatty acids and
monoacylglycerols. The fatty acids can
diffuse into the cells or exit the tissue in
the venous blood. Although LPL is a
product of a single gene in all tissues, its
transcription is regulated differently in
different tissues through the presence of
tissue-specific cis-acting elements (Braun
and Severson, 1992). For example, LPL
activity is higher in adipose tissue during
mid-gestation, and is high in mammary
gland during lactation. In cows, adipose
LPL increases markedly during mid- to late
lactation to restore energy reserves
(McNamara, 1991). During fasting, the
activity of LPL decreases in adipose tissue
and increases in the heart. Thus, LPL may
help to direct dietary fatty acids to appro-
priate tissues depending on the nutritional
state of the animal, which in turn is
signalled by insulin and other hormones.
After triacylglycerol hydrolysis, the
lipoprotein particles remaining are termed
remnants (from chylomicrons) and
intermediate-density lipoproteins (IDLs;
from VLDLs). Continued triacylglycerol
hydrolysis by LPL and, in some species,
hepatic lipase, eventually decreases the
size of the particles so that they can be
removed by the liver. Remnants and IDLs
are actively removed by the liver in most
species via interaction with the apo-B,E
receptors (Hussain et al., 1996). Other
portions of IDLs are converted to LDLs,102 J.K. Drackley