Agriculture Reference
In-Depth Information
The final steps of digestion of food proteins to absorbable peptides and free amino
acids take place at the lumenal membrane of small intestinal enterocytes (Sterchi
and Woodley, 1980a, 1980b; Sterchi, 1981; Rawlings et al., 2008). The lumenal brush
border membrane anchors a series of peptidases, most of which hydrolyze terminal
amino acids (Table 2.1). Enterokinase has the specialized function of activating the
secreted pancreatic proteases described. It has a younger phylogeny than the remain-
ing enzymes. The remaining peptidases are expressed in many tissues, including T
cells, and are identified by CD antigen numbers. These membrane enzymes often
play a role in the regulation of peptide hormone blood levels and are targets for
pharmacologic inhibitors. γ-Glutamyl peptidase is a key enzyme in the glutathione
cycle and plays a critical role in xenobiotic detoxification. The last five enzymes
diverged before bacterial emergence (2.5 bya) and likely are descendants from
membrane-bound digestive genes expressed by the first common ancestor of life (de
Duve, 2007).
L i P i D D i g e s t i o n
A surrounding membrane contributes to the defiance of entropy by the living cell.
This is a planar structure composed of phosphodiglycerides oriented to display
hydrophilic PO 4 external extensions coating a core of hydrophobic diglyceride tails
(Pollard et al., 2008, p. 113). Other lipids are also embedded into the external faces,
and hydrophobic membrane proteins and transporters traverse the lipid core. The
membrane is stable and relatively impermeable to ions and electrons. It is believed
that the earliest life forms evolved the lipid membrane to decrease local entropy
within a living cell about 3.5 bya. All sequenced genotypes have conserved enzymes
that catalyze the synthesis of coenzyme A and mevalonate in the synthetic pathway
to phosphodiglycerides and cholesterol (Friesen and Rodwell, 2004). By contrast,
only eukaryotes synthesize neutral triglycerides, which are stored as intracellular
hydrophobic droplets (Turkish and Sturley, 2007). These membrane and stored lipid
classes are major contributors to food negative entropy. The disorders of atheroscle-
rosis and obesity are thought to be influenced by quality and quantity of lipids in
foods (see Chapter 6). The glycerides are not primary products of DNA transcrip-
tion, as are proteins, but are products of regulated multienzyme metabolic pathways;
the consequence is the synthesis of a family of di- and triglycerides with fatty acids
ranging from 14 to 20 carbons in length with variable degrees of saturation.
The adult Western diet contains about 100 g/day of fat, of which more than 90%
exists as triglycerides. Virtually all food triglycerides and diglycerides require lume-
nal small intestinal digestion before more than 95% absorption (Lowe 1997, 2002).
This is accomplished by a series of lipase enzymes. Lipases are esterases that can
hydrolyze acyltriglycerides into di- and monoglycerides, glycerol, and free fatty acids
at a water-lipid interface. Gastric lipase is the initial digesting activity hydrolyzing
position 3 (sn-3) ester linkages of the glycerides. This is a developmentally important
enzyme for normal young human infants, who have a physiological delay in matura-
tion of pancreatic lipase. The human enzyme is transcribed from chromosome 10,
where it resides within a cluster of five paralogous genes. This gastric lipase chro-
mosomal grouping is conserved in rodents. The gastric expressed gene is only found
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