5. ONIUM COMPOUNDS
227
as a substrate, catechol-O-methylpherase and the adenosylmethioninecleaving enzyme of yeast, exhibit the same specificity for the steric configuration about the sulfonium center (134) (Table VII). Furthermore,
it might be noted that the enzymes which synthesize the nucleoside, of
which two have been studied in detail, possess similar steric requirements, as indicated by the fact that the optical rotation of S-adenosyl-Lmethionine is identical whether it is formed by the methionine-activating
enzyme of rabbit liver or of yeast.
TABLE VII
RELATIONSHIP BETWEEN THE CONFIGURATION OF £-ADENOSYLMETHIONINE
AND ITS BIOLOGICAL UTILIZATION«
Substrate
6
configuration
I
II
III
I
II
III
I
II
Micromoles
0.30
0.30
0.31
0.048
0.051
0.052
2.06
4.12
Methylpherase
Guanidinoacetate
Guanidinoacetate
Guanidinoacetate
Catechol-O-methyl
Catechol-O-methyl
Catechol-O-methyl
Cleaving enzyme of yeast
Cleaving enzyme of yeast
Per cent
utilization
97
49
0
100
52
0
100
47
° From data by G. de la Haba, G. A. Jamieson, S. H. Mudd, and H. H. Richards,
/. Am. Chem. Soc. 81, 3975 (1959).
6 1 = ( —)-AS-adenosyl-L-methionine, prepared enzymatically with methionine-activating enzyme of rabbit liver.
II = (±)-$-adenosyl-L-methionine, prepared by chemical methylation of #-adenosyl-L-homocysteine.
Ill = (+)-£-adenosyl-L-methionine, prepared from II by enzymatic resolution with
guanidinoacetate methylpherase followed by reisolation.
By contrast thetin-homocysteine methylpherase from horse liver does
not exhibit absolute optical specificity toward the sulfonium center inasmuch as 90% of a racemic substance, methylethyl acetothetin, was
utilized as substrate.
Finally it is of interest to discuss another aspect of the steriospecificity of methylpherases, namely specificity with respect to the steric
configuration about the α-carbon. Only one case has been studied,
guanidinoacetate methylpherase which has recently been found to catalyze methyl transfer from ( —)-S-adenosyl-D-methionine but at a rate
considerably smaller than from the L-isomer (134).
The enzymatic reactions in which S-adenosylmethionine participates
may be readily understood in terms of the chemical properties conferred
on the molecule by the presence of a positively charged sulfur atom.
227
as a substrate, catechol-O-methylpherase and the adenosylmethioninecleaving enzyme of yeast, exhibit the same specificity for the steric configuration about the sulfonium center (134) (Table VII). Furthermore,
it might be noted that the enzymes which synthesize the nucleoside, of
which two have been studied in detail, possess similar steric requirements, as indicated by the fact that the optical rotation of S-adenosyl-Lmethionine is identical whether it is formed by the methionine-activating
enzyme of rabbit liver or of yeast.
TABLE VII
RELATIONSHIP BETWEEN THE CONFIGURATION OF £-ADENOSYLMETHIONINE
AND ITS BIOLOGICAL UTILIZATION«
Substrate
6
configuration
I
II
III
I
II
III
I
II
Micromoles
0.30
0.30
0.31
0.048
0.051
0.052
2.06
4.12
Methylpherase
Guanidinoacetate
Guanidinoacetate
Guanidinoacetate
Catechol-O-methyl
Catechol-O-methyl
Catechol-O-methyl
Cleaving enzyme of yeast
Cleaving enzyme of yeast
Per cent
utilization
97
49
0
100
52
0
100
47
° From data by G. de la Haba, G. A. Jamieson, S. H. Mudd, and H. H. Richards,
/. Am. Chem. Soc. 81, 3975 (1959).
6 1 = ( —)-AS-adenosyl-L-methionine, prepared enzymatically with methionine-activating enzyme of rabbit liver.
II = (±)-$-adenosyl-L-methionine, prepared by chemical methylation of #-adenosyl-L-homocysteine.
Ill = (+)-£-adenosyl-L-methionine, prepared from II by enzymatic resolution with
guanidinoacetate methylpherase followed by reisolation.
By contrast thetin-homocysteine methylpherase from horse liver does
not exhibit absolute optical specificity toward the sulfonium center inasmuch as 90% of a racemic substance, methylethyl acetothetin, was
utilized as substrate.
Finally it is of interest to discuss another aspect of the steriospecificity of methylpherases, namely specificity with respect to the steric
configuration about the α-carbon. Only one case has been studied,
guanidinoacetate methylpherase which has recently been found to catalyze methyl transfer from ( —)-S-adenosyl-D-methionine but at a rate
considerably smaller than from the L-isomer (134).
The enzymatic reactions in which S-adenosylmethionine participates
may be readily understood in terms of the chemical properties conferred
on the molecule by the presence of a positively charged sulfur atom.