Biomedical Engineering Reference
In-Depth Information
on SDS-PAGE), had a pI of 4.8 and was optimally active at pH 6.0 (Lau
1987). In the rat, the proximal and distal forms of β-galactosidases differ
electrophoretically prior to treatment with neuraminidase. Both forms
had pH optima at pH 6.0 and were stable at temperatures up to 45°C
(Cousineau and Green 1980). A β-D-galactosidase purifi ed from rabbit
spleen had a molecular weight of 75 kDa and pI of 6.7; maximal activity
was observed at pH 4.0 (Rodriguez-Berrocal 1988). In chicken liver, a
protein of relative molecular weight greater than 200 kDa was observed
(67 kDa in the reduced form) which exhibited β-galactosidase activity.
This enzyme was active against lactose, Gal-β-1,3-Gal, Gal-β-1,6-Gal and
Gal-β-1,3-Ara (Javeri 1984).
MICROBIAL
β
-GALACTOSIDASES
Bacterial Sources
The β-galactosidase from
Corynebacterium murisepticum
was a dimer of
identical 100 kDa subunits. The K
m
value for lactose was 16.7 mM and
for 2-nitrophenyl-β-D-galactopyranoside (2NPG) was 4.4 mM (Priyolkar
et al. 1989).
B. circulans
produces three isoforms of β-galactosidases with
M
r
values of 212 kDa, 145 kDa and 86 kDa respectively. The K
m
values for
lactose exhibited by the three isoforms were 3.7 mM, 2.94 mM and 2.71
mM, whereas with 2NPG, the K
m
values were 3.6 mM, 5.0 mM and 3.3
mM, respectively (Vetere and Paoletti 1998). The β-galactosidase from
B.
macerans
has a molecular weight of 320 kDa, consisting of 78 kDa subunits
and a pI of 4.4. Optimal pH was observed at pH 6.5 and the enzyme
was stable at temperatures up to 37°C (Miyazaki 1988). A thermophilic
β-galactosidase was purified from the archaebacterium
Sulfolobus
solfatorius
. This 240 kDa tetramer had a pI of 4.5 and was optimally active
at pH 6.5 and 90°C. The K
m
for lactose was 13 mM (Pisani et al. 1990).
Psychrotrophic
Arthrobacter
isolates B7, D2 and D5 produced intracellular
β-galactosidases with temperature optima of 30-35°C, 25°C and 30-35°C,
respectively. The activity of the latter enzymes decreased within a few
minutes at 50°C (Loveland 1994).
Fungal Sources
Fungal β-galactosidase sub-units are approximately 130 kDa in size but
the active enzymes may be composed of more than one sub-unit, e.g.
the
Thermomyces lanuginosus
and
Rhizomucor
sp. enzymes are dimers
(Fischer 1995); (Shaikh et al. 1999), the
Penicillium chrysogenum
enzyme is
a tetramer (Nagy et al. 2001), while the
Aspergillus nidulans
β-galactosidase
