Acylation and carbamylation of equine muscle carbonic anhydrase (CA-III) upon reaction with p-nitrophenyl esters and carbamoyl phosphate

Reactions of the melatonin metabolite N(1)-acetyl-5-methoxykynuramine with carbamoyl phosphate and related compounds

N-[2-(6-methoxyquinazolin-4-yl)-ethyl] acetamide (MQA) is a compound formed from the melatonin metabolite N(1)-acetyl-5-methoxykynuramine (AMK). We followed MQA production in reaction systems containing various putative reaction partners, in the absence and presence of hydrogen peroxide and/or copper(II). Although MQA may be formally described as a condensation product of either N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK) with ammonia, or AMK with formamide, none of these combinations led to substantial quantities of MQA. However, MQA formation was observed in mixtures containing AMK, hydrogen peroxide, hydrogen carbonate and ammonia, or AMK, hydrogen peroxide, copper(II) and potentially carbamoylating agents, such as potassium cyanate or, more efficiently, carbamoyl phosphate. In the presence of hydrogen peroxide, copper(II) and carbamoyl phosphate, MQA was the major product obtained from AMK, but the omission of copper(II) mainly led to another metabolite, 3-acetamidomethyl-6-methoxycinnolinone (AMMC). This was caused by nitric oxide (NO) generated under oxidative conditions from carbamoyl phosphate, as shown by an NO spin trap. MQA formation with carbamoyl phosphate was not due to the possible decomposition product, formamide. The reaction of AMK with carbamoyl phosphate under oxidative conditions, in which inorganic phosphate and water are released and which differs from the typical process of carbamoylation via isocyanate, may be considered as a new physiological route of MQA formation.

Immunocytochemical localisation of carbonic anhydrase isozyme III in equine skeletal muscle

The location of carbonic anhydrase III (CA-III) in frozen sections of biopsies of Thoroughbred horse skeletal muscle was studied. Fibre types were determined by ATP-ase and succinate dehydrogenase staining. CA-III isozyme was detected using a peroxidase conjugated anti-CA-III antibody. CA-III was found to be localised in slow twitch oxidative fibres (ST), but was also present in fast twitch oxidative (FTH) fibres in small amounts. Fast twitch glycolytic (FT) fibres were stained lightly compared with control sections. The concentrations of CA-III in muscle and liver were 70 micrograms/mg protein and 4 micrograms/mg protein, respectively. CA-I and CA-II were not found in muscle extracts by the double immunodiffusion method.

Effects of carbamoylating agents on tumor metabolism

Carbamoylation of macromolecules occurs by the displacement of hydrogen on several groups, but the most stable addition at neutral pH is on amino groups. This reaction occurs predominantly with proteins and results from the administration in vivo of inorganic cyanate or organic isocyanates. The latter act more rapidly, but also are more rapidly hydrolyzed in aqueous solution. This instability has been a factor limiting study of the pharmacological properties of organic isocyanates. However, organic isocyanates are released from some nitrosoureas of value in cancer therapy such as 1,3-bis(2-chlorethyl)-1-nitrosourea (BCNU) and 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU). The carbamoylating activities of BCNU and CCNU are generally considered less significant than their alkylating activity in the action of these drugs on tumors, but carbamoylation may serve to inhibit DNA repair. There is evidence that carbamoylating agents can exert selective inhibitory effects on metabolite uptake and macromolecular synthesis in neoplastic tissues. Such selectivity is much more notable in vivo than in vitro. In the case of cyanate, the selectivity in vivo has been variously attributed to a requirement for metabolic activation, to selective effects on circulation in solid tumors, and to diminished pH in tumors. It is the distinction between such factors and the identification of critical cellular targets which provide major challenges in present studies on the effects of carbamoylating agents on tumor metabolism.

Carbonic anhydrases

Some of the current studies of carbonic anhydrases are directed to the genetic mechanisms underlying their synthesis. Determination of the structure of their genes will probably most readily resolve the question of whether the membrane bound forms of the enzyme represent products of additional loci other than those of the three well-known soluble forms. Extensions of our knowledge of the sequences of these isozymes as well as those from lower animals and from plants will make possible a more precise evaluation of the extent of the multigene aspects of these proteins and their evolutionary backgrounds. Studies of the interrelationships of the regulation of the transcriptional and translational processes of the well-known isozymes and in particular the effects of hormones will be of interest. Insights into modifications of the isozymes' synthetic processes occurring in various diseases should also be forth-coming from these studies. In addition to the above the applications of what are perhaps today somewhat classical methods of protein chemistry will be needed to explore the reasons for the changes in activity accompanying the sequence variations of the different isozymes, the decreases or increases in activity accompanying derivatizations of specific residues and the reasons for the differences in the activity of different inhibitors on the various isozymes. The broad specificity of these enzymes for different substrates and the ability of CA-III to hydrolyze various phenyl esters and in some cases to become derivatized also present problems in protein structural chemistry. In terms of the latter reactions, the meaning of the relationships of these activities to those of the protein ubiquitin, which is homologous to CA-III, needs clarification. It would appear that various of the protein structural studies will be aided by crystallographic investigations of not only CA-III but of various of its derivatives which undergo either increases or decreases in activity. The above areas of studies present a wide variety of problems for workers in various disciplines and backgrounds who are interested in the carbonic anhydrases.