Handbook of Meat Processing

88 Chapter 4

fi ed within the lysosome. On the other hand,

lysosomes have fragile membranes that may

rupture with the decreasing temperature

and pH in the muscle during postmortem

storage. Only cathepsins B, D, H, and L have

been found to degrade the same proteins

as observed during postmortem aging

(Etherington 1984 ; Etherington et al. 1987,

1990 ; Prates 2002 ; Sentandreu et al. 2002 ; Li

et al. 2008 ). Inhibition of cathepsins B and

L, however, does not prevent tenderization

(Hopkins and Thompson 2001 ). Cathepsin B

degrades myosin, and actin to a lesser extent,

while cathepsin D degrades both actin and

myosin into small peptide fragments.

Cathepsin L acts on myosin, actin, α - actinin,

troponin - T, and troponin - I. Both an endo -

and exopeptidase, cathepsin H is reported to

act on myosin (Allen and Goll 2003 ).

Although cathepsins probably do not account

for the bulk of postmortem proteolysis during

aging, their contributions cannot be com-

pletely discounted.

Calpains

Calpains are Ca 2+ - dependent cysteine prote-

ases with optimal activity at neutral pH and

are found in all kinds of living organisms,

including animals, plants, fungi, and bacteria.

Discovered about a decade after cathepsins

(Guroff 1964 ), calpains have been exten-

sively researched and the subject of many

reviews on muscle proteolysis (Goll et al.

1992, 1998, 2003 ; Geesink et al. 2000 ; Ilian

et al. 2001 ; Friedrich and Boz ó ky 2005 ;

Geesink et al. 2005 ; Koohmaraie and Geesink

2006 ; Geesink and Veiseth 2009 ). Currently,

at least 15 different calpains have been iden-

tifi ed in mammals (Suzuki et al. 2004 ). Six

different calpains are expressed as mRNA in

the mammalian skeletal muscle, but only μ -

and m - calpains and p94/calpain 3 isoforms

can be detected at the protein level (Sorimachi

et al. 1990 ; Spencer et al. 1995 ; Sorimachi

and Suzuki 2001 ; Huang and Wang 2001 ).

The Ca 2+ requirements, optimum pH and

cytoskeletal proteins. Thus, the purpose of
this chapter is to provide an overview of the
established theory of enzymatic aging tender-
ization and to review recent developments
that contribute to a more complete under-
standing of the underlying mechanisms that
infl uence postmortem proteolysis and the
aging tenderization of meat.

Enzyme Systems

Skeletal muscle contains numerous enzymes
that drive various metabolic pathways in the
living tissue. Many of these enzyme systems
are thought to remain active in postmortem
muscle and infl uence meat - quality develop-
ment. In particular, the cathepsin, calpain,
and proteasome enzyme systems have been
extensively studied to determine their roles
in the proteolysis associated with the aging
tenderization of meat.

Cathepsins

Cathepsins are acid proteases usually located
in the lysosomes (DeDuve et al. 1955 ) and
in phagocyte cells but have also been found
in the sarcoplasmic reticulum of muscle
cells (Allen and Goll 2003 ). Ultrastructural
studies indicate lysosomes are prevalent in
fetal muscle tissue but occur much less fre-
quently in adult skeletal muscle. Cathepsins
are distinguished by their active sites (aspar-
tic, cysteine, and serine proteases) and
substrate specifi city. Over 15 lysosomal
cathepsins have been identifi ed, but only
eight (B, L, H, S, F, K, D, E) have been found
in skeletal muscle fi bers (Barnier 1995 ;
Hopkins and Thompson 2002 ; Sentandreu et
al. 2002 ).
Cathepsins often are not considered as
important in meat tenderization because their
membrane - bound location is thought to limit
substrate accessibility. Lysosomes are inca-
pable of engulfi ng the myofi bril structure and
no myofi brillar fragments have been identi-