A Study of Rabbit Microsomal Esterases

Abstract

1. The microsomal fraction of rabbit liver and kidney preparations (and probably intestinal mucosa) contained a high proportion of the total esterase activity of these organs. All the substrates used confirmed this conclusion. 2. The pH optimum for the microsomal esterase preparation was 8.8-8.9 No NADPH-dependent ‘oxidative cleavage’ was observed. 3. On a total tissue-weight basis the liver had the greatest hydrolytic capacity, followed by the intestinal mucosa. The hydrolytic capacity of the blood was low compared with the liver and intestine. 4. The microsomal esterases were solubilised using Triton X-100. Sodium deoxycholate was almost as satisfactory and sonication less so. 5. Five esterases were separated electrophoretically from the solubilised liver microsomes. They could be divided into two groups on the basis of their migration rates, zone 1, relatively slow-moving, and zone II, relatively fast-moving. The band (zone 1) esterase had approximately twice the molecular weight of the bands 1-4 esterases. 6. The kidney microsomes have three major esterases which are also divided into two zones on the basis of migration rates. Mucosal microsomes also had three major esterases but a slow-moving zone was absent. 7. Generally the liver microsomal esterases had wide substrate specificity but the band 5 enzyme hydrolysed acetylsalicylic acid with difficulty whereas the band 3 enzyme hydrolysed it with comparative case. 8. None of the major microsomal esterases hydrolysed acetylcholine. 9. The use of modifiers (activators and inhibitors) for classifying the esterases gave inconclusive results. 10. The differences in the p values for the liver and plasma esterases plus differences in other properties indicated that the two groups of esterases are not the same and that the plasma esterases are probably not derived from the microsomes. 11. Liver damage, induced by carbon tetrachloride, gave rise to an increase in total plasma esterase activity. It was shown that this increase was due, mainly, to the presence of liver microsomal esterases. 12. After dosing with DDT (a well-known inducing agent) rabbit plasma hydrolysed two substrates more readily but this could not be related to the induction of microsomal esterases. No microsomal esterases appeared to have ‘leaked’ into the plasma. 13. Rabbit microsomes hydrolysed acetanilide several times faster than phenacetin whereas sheep microsomes hydrolysed both substrates equally well. Solubilised sheep liver microsomes did not hydroclyse acetyl- salicylic acid, in contrast to the rabbit. Dog liver also hydrolysed acetanilide and phenacetin equally well, but very slowly compared with the sheep and rabbit. Dog intestine was unable to hydrolyse either compound, in marked contrast to the rabbit intestine which hydrolysed them equally as well as the liver.