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D. Lemos et al.rAquaculture 186 (2000 89–105 91
1992 ., which may negatively affect the quality of postharvest shrimp by deterioration of
muscle protein during storage and processing ŽKawamura et al., 1981; Baranoski et al.,
1984 .. Since diet may affect shrimp enzyme activity Le Moullac et al., 1996; Ezquerra
Ž
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et al., 1997b; Lemos and Rodrıguez, 1998 ., studies on proteolytic enzymes may
contribute to improve the postharvest quality of shrimp.
Shrimp farming is, for many countries, a growing economical activity ŽLester, 1992 .
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To date, the choice for culturing a given species has mostly depended on known yields.
Many farmers have invested in growing species that exhibit a satisfactory performance,
regardless of whether the species were native to the area where they would be cultured.
On the other hand, culturing autochthonous or native species may be a source of
business diversification and high yields can be reached due to their inherent physio-
logical adaptation to the local environmental conditions. Culture techniques for many
autochthonous species are still not established, making attempts for cultivation a risky
practice subject to low yields. The success of rearing alternative species depends mostly
on the knowledge of their biology including nutrition. In this scenario, the availability of
high nutritional quality feeds is a crucial factor. Studies on shrimp digestion may be
helpful for improving the formulation of feeds in order to promote appropriate growth
performances ŽJones et al., 1997 . The present article describes three different methods
.
that have been used to assess protein digestion in penaeid shrimp: Ž. 1 detection and
characterization of proteinase activity, and proteinaceous proteinase inhibitors by sub-
strate- wsodium dodecyl sulphate polyacrylamide gel electrophoresis x ŽSDS-PAGE ; 2
. Ž .
quantification of proteinase activity; and Ž. 3 in vitro evaluation of digestibility of dietary
protein sources by shrimp proteinases. Previously reported data along with unpublished
data were used as examples of the utility of such techniques. The generic level taxa used
in referring the species follow the recently proposed nomenclature for the members of
´
the family Penaeidae ŽPerez-Farfante and Kensley, 1997 .
.
2. Methods to assess shrimp proteinases and protein digestion

This section presents a description of some methods to assess enzymatic protein
digestion in penaeids. Enzyme extracts may be obtained from the protein fraction of
digestive gland, depending on the size of the organism, or from whole individuals as
larvae or postlarvae. To enable the comparison of results, individuals must be sampled at
the same stage of the molt cycle ŽLovett and Felder, 1990; Fernandez et al., 1997 .
´
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Intermolt seems to be the best physiological stage, comprising from ca. 7.5% to 30% of
the total molt period since the last molt ŽDall et al., 1990 . The homogenization and
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separation of the enzyme extract must be done at about 58C. Tissues are homogenized in
distilled water and centrifuged for 20 min at 10,000=g. After the elimination of tissue
debris and lipid, the water soluble extract is the enzyme preparation for activity assays.
Proteinases, also called endopeptidases, is an important group among the digestive
enzymes of shrimp, they include trypsin and chymotrypsin which are responsible for
more than 60% of total protein digestion in penaeids ŽGalgani et al., 1984, 1985; Tsai et
al., 1986 .. Proteinases can be characterized after separation of the enzyme extract by
SDS-PAGE ŽLaemmli, 1970 , using 12% acrylamide.
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