The suicide inactivation of ox liver short chain acyl coenzyme a dehydrogenase by propionyl coenzyme a formation of an fad adduct
Shaw, L.; Engel, P.C.
Biochemical Journal 230(3): 723-732
ISSN/ISBN: 0264-6021 Accession: 006775713
1. Propionyl-CoA gave an unexpectedly low turnover (0.03 s-1) and high Km (153 .mu.M) as a substrate for ox liver short-chain acyl-CoA dehydrogenase (SCAD). 2. On addition of an excess of propionyl-CoA to SCAD the flavin A448 decreased to about 30% of its original value and the peak at 368 nm was replaced by one at 335 nm. The decrease in A448 exhibited first-order kinetics and correlated with a first-order decrease in the enzyme's catalytic activity to 22% of the initial value. 3. The flavin, released from propionyl-CoA-treated enzyme with trichloroacetic acid, reacted with O2 to form a stable free radical. This suggests that a reduced N-5 flavin adduct is formed on the enzyme and protected from O2. The released adduct was separated from unmodified flavin and excess propionyl-CoA by h.p.l.c., and was shown by 3H-labelling to contain CoA. 4. The incompleteness of the decrease in the enzyme's A448 and specific activity on incubation with propionyl-CoA probably reflects an equilibrium between covalently and non-covalently bound acyl-CoA, since the spectral changes could be reversed. The enzyme was also re-activated by dilution and incubation with a large molar excess of butyryl-CoA. The rate constant, approx. 2 .times. 10-3s-1, for re-activation, taken with the extrapolated rate constant for the opposing inactivation reaction, 8.9 .times. 10-3s-1, explains the 22% residual activity at equilibrium. 5. The results suggest that propionyl-CoA is a suicide inhibitor for SCAD.