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EC number: 200-820-0
CAS number: 74-89-5
of MA metabolism determined by colorimetric and radiochemical assay:
Mean (± S.E.) specific enzyme activities
for the group were 281 ± 32 nmol H2O2 (mg protein)/1 h for the
colorimetric assay and 366 ± 31 nmol MA metabolized (mg protein)/1 h
for the radiochemical assay.
2.Effects of clorgyline and
semicarbazide upon MA metabolism in rat aorta, human umbilical artery
The deamination of both MA and BZ was
largely resistant to inhibition by clorgyline concentrations up to 1
mM, whereas similar concentrations of semicarbazide produced a
progressive degree of inhibition which resulted in virtually complete
inhibition of MA metabolism, and around 80% inhibition of BZ
metabolism at 1 mM semicarbazide. These results suggest that MA is
predominantly, if not exclusively, a substrate for the soluble amine
oxidase in human plasma.
3. Kinetic constants for MA metabolism
in rat aorta, human umbilical artery and plasma
Mean values (± S.E.) from four different
samples were: Km(µM) of 516 ± 74 (MA) and 225 ± 36
(BZ): Vmax (nmol (mL serum)/1 h) of 48 ± 5 (MA) and 28 ± 3(BZ).
4. Inhibition of [14C]MA metabolism in
human umbilical artery and plasma by unlabelled BZ
BZ was found to be a competitive inhibitor
of MA metabolism. Mean values (± S.E.) for Ki (µM) from
experiments with different samples of each enzyme source were 220 ±10
(umbilical artery, n=3) and 172 ± 27 (plasma, n=4).
5. Kinetic constants for (14C]BZ
metabolism in human umbilical artery and product formation from mixtures
of[14C]BZ and [14C]MA
200 µM [14C]BZ and 800 µM [14C]MA
were cocultivated with homogenates of human umbilical artery. The
substrate concentrations were chosen to approximate closely to their Km
values-found in the earlier work reported in the original paper. If a
single enzyme metabolizes two different substrates at the same catalytic
site, the product formation calculated (for detailed calculations see
original paper) should be comparable with that experimentally
determined. Table 1 shows the results obtained in this experiment:
Table 1. Comparison of predicted and actual metabolite formation in mixtures of [14C]BZ and [14C]MA.
Ratio (Actual vT/ Predicted vT)
Mean ratio= 1.04 ± 0.03
vAand vBrepresent [14C]metabolite formation (in d min-1) from 200 µMBZ and 800 µMMA, respectively, in assays containing the appropriate amine individually. The predicted metabolite formation (vT) from a mixture of the two amines if metabolized by the same enzyme was determined as described in the text, and compared with actual experimentally determined values. Experiments were carried out on 4 different arteries, assays on each artery involving concurrent determinations (in triplicate) of BZ and MA metabolism alone, and in a mixture.
of β-aminopropionitrile (BAPN) upon MA metabolism in human umbilical
BAPN was found to be a competitive
inhibitor of MA metabolism
Lyles et al. investigated in 1990 the
metabolism of methylamine hydrochloride in vitro. An ion exchange
radiochemical assay has been developed to study the deamination of
[14C]methylamine (MA) in homogenates of rat aorta and human umbilical
artery, as well as in samples of human plasma. MA metabolism was found
to be inhibited almost completely by 1 mM semicarbazide, but virtually
unaffected by 0-1 mM clorgyline, suggesting that MA is a substrate for
the semicarbazide-sensitive amine oxidase (SSAO) activities which also
metabolize benzylamine (BZ) in these sources. Mean Km values for MA
metabolism by aorta, umbilical artery and plasma were 182, 832 and 516 µM,
respectively, with corresponding Vmax values in aorta and umbilical
artery of 100 and 590 nmol /(mg prot.) h, and in plasma of 48 nmol /(mL
serum) h. Kinetic constants determined for [14C]BZ metabolism in plasma
(by an organic solvent extraction assay) and in umbilical artery (by the
ion exchange assay) yielded mean Km values of 225 µM (plasma), 222 µM
(umbilical artery), and Vmax values of 28 nmol (mL serum)/ 1 h (plasma)
and 377 nmol /(mg prot.) h (umbilical artery). The deamination of
[14C]MA was inhibited competitively by unlabelled BZ, with Ki values in
umbilical artery and plasma of 220 and 172 µM, respectively. Also,
metabolite formation from mixtures of [14C]BZ (200 µM) and [14C]MA (800
µM) was extremely close to that predicted for a single enzyme capable of
metabolizing two alternative substrates in a competitive fashion.
β-Aminopropionitrile was found to be a reversible, competitive inhibitor
(Ki of 165 µM) of [14C]MA metabolism in umbilical artery, inhibitory
properties characteristic of those found previously for the effects of
β-aminopropionitrile upon BZ-metaboIizing SSAO activities in other
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