Allantoinase hydrolyzes allantoin, a purine metabolite and a nitrogen trans
port molecule in plants, to form allantoic acid. The standard enzyme assay
involves acid-catalyzed product decomposition to form urea and glyoxylate,
reaction of glyoxylate with phenylhydrazine, and oxidative conversion of ph
enylhydrazone to 1,5-diphenylformazan that is measured colorimetrically. Wh
en used with crude cell extracts this assay is problematic and its complexi
ty is a hindrance to detailed enzyme characterization; thus, three alternat
ive assays were developed. In the first assay, 2,4-dinitrophenylhydrazine w
as reacted with allantoate-derived glyoxylate and the concentration of hydr
azone was measured directly by its absorbance at 450 nm. This assay exhibit
ed enhanced reproducibility compared to the standard method and entailed fe
wer steps, but was 3-fold less sensitive. The second assay combined allanto
ate decomposition and glyoxylate reaction with o-phenylenediamine to yield
a quinoxalone that was detected by its absorbance at 340 nm. This one-step
method was the least error prone of those examined, but was more than 10-fo
ld less sensitive than the standard assay. The third assay involved urease-
catalyzed hydrolysis of allantoate-derived urea, followed by reaction of th
e released ammonia to form indophenol. This was the most laborious of the a
ssays, but was more sensitive than the standard method. (C) 1999 Academic P
ress.