We have purified a novel alliinase (EC 4.4.1.4) from roots of onion (Allium
cepa L.). Two isoforms with alliinase activity (I and II) were separated b
y concanavalin A-Sepharose and had molecular masses of 52.7 (I) and 50.5 (I
I) kD on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and 57
(I) and 57.5 (II) kD by gel filtration fast-protein liquid chromatography.
Isoform 1 had an isoelectric point of 9.3, while isoform II had isoelectric
points of 7.6, 7.9, 8.1, and 8.3. The isoforms differed in their glycosyla
tion. Both contained xylose/fucose containing complex-type N-linked glycans
, and isoform II also contained terminal mannose structures. Both isoforms
had activity with S-alk(en)yl-L-cysteine sulfoxides. Unlike other allium al
liinases, A. cepa root isoforms had cystine lyase activity. We cloned a gen
e from A. cepa root cDNA and show that it codes for A. cepa root alliinase
protein. Homology to other reported allium alliinase genes is 50%. The gene
coded for a protein of mass 51.2 kD, with two regions of deduced amino aci
d sequence identical to a 25- and a 40-amino acid region, as determined exp
erimentally. The A. cepa root alliinase cDNA was expressed mainly in A. cep
a roots. The structure and function of the alliinase gene family is discuss
ed.