Food Biochemistry and Food Processing

318 Part III: Muscle Foods

lysosomes and have optimal acid pH (5.5). DPP I
has a special preference for hydrolyzing the dipep-
tides Ala-Arg and Gly-Arg, while DPP II prefers a
terminal Gly-Pro sequence. DPP III is located in the
cytosol and has special preference for terminal Arg-
Arg and Ala-Arg sequences. DPP IV is linked to the
plasma membrane and prefers a terminal Gly-Pro
sequence. Both DPP III and IV have an optimal pH
around 7.5–8.0. All these peptidases have been puri-
fied and fully characterized in porcine skeletal mus-
cle (Toldrá 2002).

Exoproteases: Aminopeptidases

There are five aminopeptidases, known as leucyl,
arginyl, alanyl, pyroglutamyl, and methionyl amino-
peptidases, based on their respective preference or
requirement for a specific N-terminal amino acid.
They are able, however, to hydrolyze other amino
acids, although at a lower rate (Toldrá 1998). Amin-
opeptidases are metalloproteases with a very high
molecular mass and complex structures. All of them
are active at neutral or basic pH. Alanyl aminopepti-
dase, also known as the major aminopeptidase be-
cause it exhibits very high activity, is characterized
by its preferential hydrolysis of alanine, but it is
also able to act against a wide spectrum of amino
acids such as aromatic, aliphatic, and basic aminoa-
cyl bonds. Methionyl aminopeptidase has preference
for methionine, alanine, lysine, and leucine, but also
has a wide spectrum of activity. This enzyme is acti-
vated by calcium ions. Arginyl aminopeptidase, also
known as aminopeptidase B, hydrolyzes basic amino
acids such as arginine and lysine (Toldrá and Flores
1998).
Carboxypeptidases are located in the lysosomes
and have optimal acid pH. They are able to release
free amino acids from the carboxy termini of pep-
tides and proteins. Carboxypeptidase A has pre-
ference for hydrophobic amino acids, while carb-
oxypeptidase B has a wide spectrum of activity
(McDonald and Barrett 1986).

LIPOLYTICENZYMES

Lipolytic enzymes are characterized by their ability
to degrade lipids, and they receive different names
depending on their mode of action (see Fig. 14.2).
They are known as lipases when they are able to
release long-chain fatty acids from tri-acylglycerols,

while they are know as esterases when they act on

short-chain fatty acids. Lipases and esterases are lo-

cated either in the skeletal muscle or in the adipose

tissue. Phospholipases, mainly found in the skeletal

muscle, hydrolize fatty acids at positions 1 or 2 in

phospholipids.

Muscle Lipases

Lysosomal acid lipase and acid phospholipase are

located in the lysosomes. Both have an optimal acid

pH (4.5–5.5) and are responsible for the generation

of long-chain free fatty acids. Lysosomal acid lip-

ase has preference for the hydrolysis of tri-

acylglycerols at positions 1 or 3 (Fowler and Brown

1984). This enzyme also hydrolyzes di- and mono-

acylglycerols but at a lower rate (Imanaka et al.

1985, Negre et al. 1985). Acid phospholipase hydro-

lyzes phospholipids at position 1 at the water-lipid

interface.

Phospholipase A and lysophospholipase have op-

timal pH in the basic region and regulate the hydrol-

ysis of phospholipids, at positions 1 and 2, respec-

tively. The activities of these enzymes are higher in

oxidative muscles than in glycolytic muscles, and

this fact would explain the high content of free fatty

acids in oxidative muscles. The increase in activity

is about 10- to 25-fold for phospholipase A and four-

to five-fold for lysophospholipase (Alasnier and

Gandemer 2000).

Acid and neutral esterases are located in the lyso-

somes and cytosol, respectively, and are quite stable

(Motilva et al. 1992). Esterases are able to hydrolyze

short-chain fatty acids from tri-, di-,and monoacyl-

glycerols, but they exert poor action due to the lack

of adequate substrate.

Adipose Tissue Lipases

Hormone-sensitive lipase is the most important

enzyme present in adipose tissue. This enzyme is

responsible for the hydrolysis of stored adipocyte

lipids. It has a high specificity and preference for

the hydrolysis of long-chain tri- and diacylglycerols

(Belfrage et al. 1984). This enzyme has positional

specificity since it hydrolyzes fatty acids at positions

1 or 3 in tri-acylglycerols four times faster than it

hydrolyzes fatty acids in position 2 (Belfrage et al.

1984). The hormone-sensitive lipase has a molecu-

lar mass of 84 kDa and neutral optimal pH, around