Open AccessThiopurine Methyltransferase in Red Blood Cells of Dogs, Cats, and Horses
Author(s) -
White Stephen D.,
Rosychuk Rod A.W.,
Outerbridge Catherine A.,
Fieseler Kathryn V.,
Spier Sharon,
Ihrke Peter J.,
Chapman Phillip L.
Publication year - 2000
Publication title -
journal of veterinary internal medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.356
H-Index - 103
eISSN - 1939-1676
pISSN - 0891-6640
DOI - 10.1111/j.1939-1676.2000.tb02266.x
Subject(s) - thiopurine methyltransferase , cats , azathioprine , medicine , pharmacology , disease
Our objective was to determine if thiopurine methyltransferase (TPMT), the enzyme important in the metabolism of azathioprine in human beings, is detectable in red blood cell lysates (RBCL) of healthy dogs, cats, and horses. Values for TPMT activity were determined from blood collected from 20 healthy dogs, cats, and horses. The TPMT activity in each animal's RBCL was determined using a radioenzymatic end point involving TPMT‐facilitated metabolism of 6‐mercaptopurine to 6‐methylmercaptopurine (6‐MMP). One unit of TPMT activity represents the formation of 1 nmol of 6‐MMP per milliliter of packed red blood cells per hour of incubation at 37°C. TPMT activity in RBCL was detectable in all species, with mean RBC values ± standard deviation of 17.9 ± 3.79 U/mL in dogs; 2.76 ± 0.70 U/mL in cats; and 2.185 ± 0.36 U/mL in horses. Values for TPMT in the 3 species were significantly ( P < .05) different from one another. TPMT values for dogs were significantly higher than the other species, and TPMT values for cats were significantly higher than those for horses. We conclude that RBCL TPMT values are measurable in dogs, cats, and horses and that dogs have higher values than cats or horses. These findings are consistent with the lower tolerance for azathioprine in cats as compared with dogs. It remains to be determined whether RBCL TPMT values in these species correlate with TPMT activity in the liver, where most of the metabolization of azathioprine is believed to occur.