Biomedical Engineering Reference
In-Depth Information
ENDOGENOUS PLANT
β
-GALACTOSE-RELEASING
ENZYMES
Endogenous galactosidase/galactanase activity has been detected in
plants and is evident from the loss in cell wall galactosyl residues observed
in fl eshy fruits during ripening (Kim et al. 1991). An exo-β-1,4-galactanase
has been isolated from
Lupinus angustifolius
(lupin) seed. The protein (60
kDa and pI 7.0) had limited exo-β-galactosidase activity but displayed
rapid exo-action on the β-1,4-galactan from lupin itself (Buckeridge 1994).
Partial purifi cation of β-galactosidases from melon (
Cucimis melo
) revealed
one isoform, which increased as ripening proceeded and was active on
pectic substances (Ranwala 1992). Levels of an exo-acting apple (
Malus
domestica
) β-galactosidase, active against β-1,4-galactan, also increased
during fruit development. In apple, two polypeptides of 44 kDa and
32 kDa were detected following SDS-PAGE (Ross et al. 1994). Three
β-galactosidases (I-III) were produced by ripening mango (
Mangifera
indica
) fruit, all of which exhibited galactanase activity. Optimum
activity was noted at pH 3.2 for β-galactosidase I and II and at pH 3.6 for
β-galactosidase III (Ali et al. 1995). Tomato fruit (
Lycopersicon esculentum
)
produced a β-galactosidase with exo-1,4-β-D-galactanase activity during
ripening. A 75 kDa enzyme with a pI of 9.8 was identifi ed, which was
optimally active at pH 4.5 and released galactose, thus establishing its exo-
activity. The nucleotide sequence of tomato β-galactosidase demonstrated
homology with the sequences of β-galactosidases from apple (Carey 1995)
and papaya (Othman 1998), as well as with the SR12 cDNA clone from
senescing carnation (
Dianthus caryophyllus
) where loss of cell wall galactose
has also been reported (De Vetten 1990). β-Galactosidase activity has also
been detected in almond (
Prunis dulcis
), barley (
Hordeum vulgare
), carrot
(
Daucus carota
), french bean (
Phaseolus vigna
),
Petunia
sp.,
Trifolium
sp. and
rice (
Oryza sativa
) amongst others (Dey 1984).
MAMMALIAN
β
-GALACTOSIDASES
Mammals require β-galactosidase to digest lactose, a disaccharide found
in milk and other dairy foodstuffs. Lactose-intolerant individuals, do
not produce β-galactosidase and suffer a range of complaints as a result
of ingesting milk products. Purifi ed human intestinal β-galactosidase,
which has a pH optimum of pH 5.8 to pH 6.0, was purifi ed to apparent
homogeneity. In its amphiphilic form it is composed of two 160 kDa
monomers. The membrane anchoring segment and bound detergent had
a molecular weight of approx. 40 kDa (Skovbjerg et al. 1981). Another
human intestinal lactase, with a M
r
of 380 kDa by gel fi ltration (or 170 kDa
