In research of serum lactate dehydrogenase (S-LDH) isoenzyme for clinical application to racehorses, the first attempt was made to clarify the normal distribution of LDH Isoenzyme in the organs of 1 healthy horse and in the sera of 19 healthy horses by means of YOSHIDA's detecting method of LDH Isoenzyme. Besides, studies were made on changes in S-LDH isoenzyme in 2 horses intoxicated with carbon tetrachloride, 3 horses experimentally infected with equine infectious anemia (EIA), and a horse infected with EIA (chronic type) in the field. Discussion was made especially on the relationship between the appearance of S-LDH
5 isoenzyme and the hepatic dysfunction . The following results were obtained. 1)In the normal equine organs, LDH Isoenzyme consisted of 5 fractions. Of these fractions, LDH
1 occupied a high percentage in each tissue of erythrocyte, heart muscle, cerebellum, pancreas, and kidney, LDH
2 in salivary gland, LDH
3 in adrenal gland, spleen, lymph node, and cerebrum, and LDH
4 in thyroid gland, lung, and skeletal muscle . LDH
5 was contained exclusively in liver, skeletal muscle, and leukocyte. The lactate dehydrogenase isoenzyme contained in the tissues of the equine digestive tract was chiefly composed of LDH
3 and LDH
4. Part of the stomach (pars pylolica and pars fundica) had a high percentage of LDH
1 and LDH
2. 2)Of the fractions of S-LDH isoenzyme contained in the sera of 19 healthy horses, LDH
3 showed the highest percentage, 42.1±4.3 %, and wasfollowed by LDH
2 (34.0±4.0%), LDH
1 (19.4±4.8%), and LDH
4 (4 .2±2.0 %). Fraction LDH
5 could not be found in the serum of any horse. It was better to preserve normal equine serum in the deep-freezer than in the refrigerator. 3) The fraction of S-LDH isoenzyme were located among the serum protein fraction by the aid of relative mobility. Fraction LDH
1 was detected in the post-position of albumin, LDH
2 in the position of α
2-globulin, LDH
3 in the position of β
2-globulin (the position of application), LDH
4 in the middle position of β2 and γ-globulin, and LDH
5 in the fore-end of γ-globulin. 4)It was presumed that LDH
5 might appear during one or two days in horses intoxicated with carbon tetrachloride. When horses were administered with 0.4 ml/kg of carbon tetrachloride 9 times at a week's intervals, the relative percentage of LDH
5 and the activity of S-GOT showed a tendency to decrease after every administration. 5)In horses infected with EIA, the total activity of S-LDH increased a little late in the febrile stage. LDH
3, LDH
4, and LDH
2 increased in value in the order listed in this stage. LDH
4 showed the most characteristic increase. In horses inoculated with EIA virus, there was a tendency for LDH
5 to appear frequently in an early febrile stage (during a period from the 15th to 35th day after inoculation). In this stage, a high ratio of LDH
4 could be found. In the field case classified as chronic type, these tendencies were not so outstanding as in the inoculated cases. On the basis of these findings, it is concluded that the confirmation of LDH
5, which cannot be found in normal equine serum, may be a valuable index for the early diagnosis of hepatocellular damage in racehorses.
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