222
G. L. CANTONI
et al. (178) in pigeon liver, and by Ericson in pig liver (179). The
reaction catalyzed by this enzyme* is described by the equation:
Dimethylacetothetin + Homocysteine —> Methylmercapto acid + Methionine + H+
(21)
The most complete study of this enzyme appears to be that of Durell,
Anderson, and Cantoni who have obtained from horse liver a preparation
which may be considered essentially homogeneous as judged by physicochemical criteria (ultracentrif ugation and electrophoresis). The purified
enzyme has a molecular weight of approximately 200,000 and a turnover
number of 3000 (^moles of methionine//Amole enzyme/minute).
The value for the turnover number is considerably higher than that
estimated for the other methylpherases so far studied. The enzyme is
present in large amounts in horse liver and it may be calculated that it
is equivalent to between 1 and 2% of the total liver protein. Although as
noted above the enzyme has been found in the liver and kidney of
several vertebrate species it has not yet been detected in microorganisms, protozoa, or plants.
Since a proton is formed in the reaction, the enzyme may be assayed
manometrically (120), as is true also of other alkylpherase reactions in
which the onium structure is lost (180).
The question of the relationship of the thetin-homocysteine methylpherase to betaine-homocysteine methylpherase is not entirely resolved.
It is perhaps significant in this connection that dimethylthetin is not a
naturally occurring compound. Furthermore it may be seen from Table
VI, which describes the specificity of the purified thetin-homocysteine
methylpherase for a variety of methyl donors, that betaine will serve as a
methyl donor with this enzyme, albeit at approximately 1/200 the rate
at which dimethylacetothetin is utilized. Originally Dubnoff and Borsook
(176) showed that the enzymatic activities towards the two substrates
could be distinguished by a difference in the stability of the two enzymatic activities at acid pH. It must be emphasized, however, that a purified preparation of betaine methylpherase free of activity towards thetin
has not yet been obtained and that all preparations of the thetin enzyme
can also utilize betaine as a substitute but at great sacrifice in activity.
No evidence for the requirement of a cofactor has been conclusively
disclosed either in the purified enzymes of horse and rat liver or in the
* Since enzymatic alkyl transfer reaction from onium compounds have common
characteristics, it is suggested that uniform nomenclature be adopted and that the
enzymes be identified by the donor compound, the acceptor compound, the group
transferred, and the suffix pherase (e.g., dimethylacetothetin-homocysteine methylpherase ).
G. L. CANTONI
et al. (178) in pigeon liver, and by Ericson in pig liver (179). The
reaction catalyzed by this enzyme* is described by the equation:
Dimethylacetothetin + Homocysteine —> Methylmercapto acid + Methionine + H+
(21)
The most complete study of this enzyme appears to be that of Durell,
Anderson, and Cantoni who have obtained from horse liver a preparation
which may be considered essentially homogeneous as judged by physicochemical criteria (ultracentrif ugation and electrophoresis). The purified
enzyme has a molecular weight of approximately 200,000 and a turnover
number of 3000 (^moles of methionine//Amole enzyme/minute).
The value for the turnover number is considerably higher than that
estimated for the other methylpherases so far studied. The enzyme is
present in large amounts in horse liver and it may be calculated that it
is equivalent to between 1 and 2% of the total liver protein. Although as
noted above the enzyme has been found in the liver and kidney of
several vertebrate species it has not yet been detected in microorganisms, protozoa, or plants.
Since a proton is formed in the reaction, the enzyme may be assayed
manometrically (120), as is true also of other alkylpherase reactions in
which the onium structure is lost (180).
The question of the relationship of the thetin-homocysteine methylpherase to betaine-homocysteine methylpherase is not entirely resolved.
It is perhaps significant in this connection that dimethylthetin is not a
naturally occurring compound. Furthermore it may be seen from Table
VI, which describes the specificity of the purified thetin-homocysteine
methylpherase for a variety of methyl donors, that betaine will serve as a
methyl donor with this enzyme, albeit at approximately 1/200 the rate
at which dimethylacetothetin is utilized. Originally Dubnoff and Borsook
(176) showed that the enzymatic activities towards the two substrates
could be distinguished by a difference in the stability of the two enzymatic activities at acid pH. It must be emphasized, however, that a purified preparation of betaine methylpherase free of activity towards thetin
has not yet been obtained and that all preparations of the thetin enzyme
can also utilize betaine as a substitute but at great sacrifice in activity.
No evidence for the requirement of a cofactor has been conclusively
disclosed either in the purified enzymes of horse and rat liver or in the
* Since enzymatic alkyl transfer reaction from onium compounds have common
characteristics, it is suggested that uniform nomenclature be adopted and that the
enzymes be identified by the donor compound, the acceptor compound, the group
transferred, and the suffix pherase (e.g., dimethylacetothetin-homocysteine methylpherase ).