Cy. Wu et al., CHEMICAL MECHANISM OF THE ENDOGENOUS ARGININOSUCCINATE LYASE ACTIVITYOF DUCK LENS DELTA-2-CRYSTALLIN, Biochemical journal, 333, 1998, pp. 327-334
The endogenous argininosuccinate lyase activity of duck delta 2-crysta
llin was specifically inactivated by the histidine-specific reagent, d
iethyl pyrocarbonate. The protein was protected by L-citrulline or L-a
rginine from the diethyl pyrocarbonate inactivation. To characterize f
urther the chemical mechanism of the delta 2-crystallin-catalysed reac
tion, deuterium-labelled argininosuccinate was enzymically synthesized
from fumarate and L-arginine with delta 2-crystallin in (H2O)-H-2. Th
e argininosuccinate synthesized contained about 19% of the anhydride f
orm; however, the deuterium was clearly demonstrated to be incorporate
d enantioselectively. Only the pro-H-R atom at C-9 of the succinate mo
iety was labelled in the [H-2]argininosuccinate-9-d synthesized, which
indicates an anti-elimination mechanism for the endogenous argininosu
ccinate lyase activity of delta 2-crystallin. The enzymic activity of
duck lens delta 2-crystallin in the pH range 5.5-8.5 was investigated
using both protium- and deuterium-labelled argininosuccinate as the su
bstrate. From the log k(cat) versus pH plot, two molecular pK(a) value
s of 6.18 +/- 0.02 and 8.75 +/- 0.03 were detected in the delta 2-crys
tallin-argininosuccinate binary complex. The former must be dehydronat
ed and the latter hydronated to achieve an optimum reaction rate. The
log k(cat)/K-m versus pH plot suggested two molecular pK(a) values of
5.96 +/- 0.09 and 8.29 +/- 0.10 for the free delta 2-crystallin to be
involved in the substrate binding. Small kinetic isotope effects of 1.
17 +/- 0.02 and 1.05 +/- 0.09 were found for k(cat) and k(cat)/Km resp
ectively. Combining results from labelling and kinetic analysis indica
tes that the endogenous argininosuccinate lyase activity of duck delta
2-crystallin is compatible with a stepwise ElcB mechanism, the rate-l
imiting step probably at the C-N bond-cleavage step.