Hepatic and pulmonary enzyme activities in horses.
OBJECTIVE
To determine hepatic and pulmonary phase-I and phase-II enzyme activities in horses.


METHODS
Pulmonary and hepatic tissues from 22 horses that were 4 months to 32 years old.


METHODS
Pulmonary and hepatic tissues from horses were used to prepare cytosolic (glutathione S-transferase and soluble epoxide hydrolase) and microsomal (cytochrome P450 monooxygenases) enzymes. Rates of microsomal metabolism of ethoxyresorufin, pentoxyresorufin, and naphthalene were determined by high-performance liquid chromatography. Activities of glutathione S-transferase and soluble epoxide hydrolase were determined spectrophotometrically. Cytochrome P450 content was determined by carbon monoxide bound-difference spectrum of dithionite-reduced microsomes. Activity was expressed relative to total protein concentration.


RESULTS
Microsomal protein and cytochromeP450 contents were detectable in all horses and did not vary with age. Hepatic ethoxyresorufin metabolism was detected in all horses; by comparison, pulmonary metabolism of ethoxyresorufin and hepatic and pulmonary metabolism of pentoxyresorufin were detected at lower rates. Rate of hepatic naphthalene metabolism remained constant with increasing age, whereas rate of pulmonary naphthalene metabolism was significantly lower in weanlings (ie, horses 4 to 6 months old), compared with adult horses. Hepatic glutathione S-transferase activity (cytosol) increased with age; however, these changes were not significant. Pulmonary glutathione S-transferase activity (cytosol) was significantly lower in weanlings than adult horses. Hepatic and pulmonary soluble epoxide hydrolase did not vary with age of horses.


CONCLUSIONS
Activity of cytochrome P450 isoforms that metabolize naphthalene and glutathione S-transferases in lungs are significantly lower in weanlings than adult horses, which suggests reduced ability of young horses to metabolize xenobiotics by this organ.
Lakritz
J
J
Department of Anatomy, School of Veterinary Medicine, University of California, Davis 95616, USA.
Winder
B S
BS
Noorouz-Zadeh
J
J
Huang
T L
TL
Buckpitt
A R
AR
Hammock
B D
BD
Plopper
C G
CG
D000375:Aging / Q000378:metabolism; D000818:Animals; D019363:Cytochrome P-450 CYP1A1 / Q000378:metabolism; D019362:Cytochrome P-450 CYP2B1 / Q000378:metabolism; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004851:Epoxide Hydrolases / Q000378:metabolism*; D005982:Glutathione Transferase / Q000378:metabolism*; D006736:Horses / Q000254:growth & development / Q000378:metabolism*; D008099:Liver / Q000201:enzymology* / Q000254:growth & development; D008168:Lung / Q000201:enzymology* / Q000254:growth & development; D008861:Microsomes / Q000201:enzymology*; D008862:Microsomes, Liver / Q000201:enzymology
D003577:Cytochrome P-450 Enzyme System; D019363:Cytochrome P-450 CYP1A1; D019362:Cytochrome P-450 CYP2B1; D005982:Glutathione Transferase; D004851:Epoxide Hydrolases