Food Biochemistry and Food Processing

14 Biochemistry of Processing Meat and Poultry 319

7.0. The mono-acylglycerol lipase is mainly present
in the adipocytes, and very little is present in stromal
and vascular cells. It has a molecular mass of 160
kDa and hydrolyzes medium- and long-chain mono-
acylglycerols resulting from previous hydrolysis by
the hormone-sensitive lipase (Tornqvist et al. 1978).
Lipoprotein lipase is located in the capillary en-
dothelium and is able to hydrolyze the acylglycerol
components at the luminal surface of the endo-
thelium (Smith and Pownall 1984), with preference
for fatty acids at position 1 over those at position 3
(Fielding and Fielding 1980). Lipoprotein lipase is
an acylglycerol lipase responsible for the degrada-
tion of lipoprotein triacylglycerol. Its molecular
mass is around 60 kDa and it has an optimal basic
pH. Unsaturated mono-acylglycerols are more
quickly hydrolyzed than saturated compounds (Mil-
ler et al. 1981).
The lipolysis phenomenon in adipose tissue is not
so complex as in muscle. The hormone-sensitive
lipase hydrolyzes tri- and diacylglycerols, as a rate-
limiting step. The resulting mono-acylglycerols from
this reaction or from lipoprotein lipase (Belfrage et
al. 1984) are then further hydrolyzed by the mono-

acylglycerol lipase. The end products are glycerol

and free fatty acids.

Acid and neutral esterases are also present in adi-

pose tissue (Motilva et al. 1992). During mobili-

zation of depot lipids, esterases can participate by

mobilizing stored cholesteryl esters. Esterases can

also degrade lipoprotein cholesteryl esters taken up

from the plasma (Belfrage et al. 1984).

MUSCLEOXIDATIVE ANDANTIOXIDATIVE

ENZYMES

Oxidative Enzymes

Lipoxygenase contains iron and catalyzes the incor-

poration of molecular oxygen into polyunsaturated

fatty acids, especially arachidonic acid, and esters

containing a Z,Z-1,4-pentadien (Marczy et al. 1995).

They receive different names, 5-, 12-, or 15-lipoxy-

genase, depending on the position where oxygen is

introduced. The final product is a conjugated hydro-

peroxide. They usually require millimolar concentra-

tions of Ca
^2 , and their activity is stimulated by ATP

(Yamamoto 1992). Lipoxygenase has been found to

Figure 14.2.Mode of action of muscle lipases and phospholipases.