Alpha-methylhistamine methylation by histamine methyltransferase

1981 ◽  
Vol 11 (5) ◽  
pp. 425-428 ◽  
Author(s):  
Lindsay B. Hough ◽  
J. K. Khandelwal ◽  
Thomas W. Mittag
Keyword(s):  
Histamine Methyltransferase

scholarly journals The kinetic properties and reaction mechanism of histamine methyltransferase from human skin

1980 ◽  
Vol 187 (3) ◽  
pp. 819-828 ◽  
Author(s):  
D M Francis ◽  
M F Thompson ◽  
M W Greaves
Keyword(s):  
Reaction Mechanism ◽  
Human Skin ◽  
Substrate Inhibition ◽  
Kinetic Properties ◽  
Sequential Reaction ◽  
Methyl Group ◽  
Michaelis Constants ◽  
Histamine Methyltransferase ◽  
The Mean ◽  
Inhibited Enzyme

The substrate kinetic properties of histamine methyltransferase from human skin were studied at limiting concentrations of both histamine and S-adenosylmethionine. Substrate inhibition by histamine was observed at concentrations above 10 microM. Primary plots showed evidence of a sequential reaction mechanism. The Michaelis constants were derived from secondary plots of slopes from the primary plots ([S]/v versus [S]) versus reciprocal of the second substrate concentration. The mean Km values for histamine and S-adenosylmethionine were 4.2 and 1.8 microM respectively. Histamine in concentrations of 25-100 microM inhibited enzyme activity uncompetitively with respect to S-adenosylmethionine. No substrate inhibition was observed with S-adenosylmethionine. To elucidate the reaction mechanism further, inhibition by the two products, S-adenosylhomocysteine and 1-methylhistamine, was studied. S-Adenosylhomocysteine inhibited non-competitively with respect to histamine and competitively with respect to S-adenosylmethionine. 1-Methylhistamine inhibited non-competitively with respect to histamine and to S-adenosylmethionine. These results are interpreted as providing evidence for an ordered sequential Bi Bi reaction mechanism, with the methyl-group donor S-adenosylmethionine as the first substrate that adds to the enzyme and histamine as the second substrate. 1-Methylhistamine is the first product to leave the enzyme and S-adenosylhomocysteine is the second. The results are discussed in terms of the possible role that this enzyme could play in the modulation of histamine-mediated reactions in skin.

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