Carbonic anhydrase

Comparative QSAR of the sulfonamide function

The aromatic SO(2)NH(2) function has been characterized in biological reactions and in physical organic chemical reactions using the Hammett constant sigma, the Swain-Lupton parameter F and steric constants. The results allow comparable support for the biological processes from the much better understood mechanistic organic reactions. The approach is applied to the enzyme carbonic anhydrase and the rate of excretion of Na(+) by rats. From the study of QSAR on the ionization of the amide function, it is clearly apparent that it is the ionized form of SO(2)NH(2) that promotes the biological processes. It is clear from Fujita-Nishioka proposal that, in dealing with ortho substituents, one should routinely test sigma, F and a steric parameter to account for substituent effects.

Total body zinc depletion and its relationship to the development of hyperprolactinemia in chronic renal insufficiency

Modulation of free plasma zinc levels has been implicated in the increase in plasma prolactin levels seen in patients with chronic renal insufficiency (CRI). The relative importance of this mechanism in comparison to others, however, has not been elucidated. Zinc equilibrium between plasma and red blood cells is partly dependent upon red blood cell carbonic anhydrase (CA). In the present paper, we have investigated the interrelationships among total plasma zinc, leukocyte zinc, prolactin, and erythrocyte CA in patients with CRI. Uremic patients were shown to have significantly increased levels of plasma prolactin and erythrocyte CA activity when compared to normal controls. Moreover, red blood cell CA total concentration and isoenzyme-I and-II levels, as well as plasma zinc were found to be significantly decreased in uremic patients in comparison to normal controls. In patients with CRI, a negative correlation was demonstrated between erythrocyte CA catalytic activity and plasma zinc, as well as between plasma zinc and plasma prolactin levels. Moreover, leukocyte zinc content, which is a reliable indicator of total body zinc stores, was found to be significantly decreased in uremic patients when compared to normal controls. A strong negative correlation between leukocyte zinc content and plasma prolactin levels was documented in CRI patients. Our results suggest that alterations in erythrocyte CA levels, enzymatic activity or isoenzyme profile are most probably mechanistically and etiologically unrelated to the high plasma prolactin levels in CRI patients. Contrariwise, depletion of total body zinc stores, rather than redistribution of this trace metal among extracellular compartments, may represent one of the major contributing mechanisms leading to uremic hyperprolactinemia.

Diuretic drugs and the treatment of edema: from clinic to bench and back again

Despite wide variations in dietary NaCl intake, homeostatic mechanisms ensure that renal NaCl excretion matches intake at steady state. This does not imply, however, that extracellular fluid volume is maintained within narrow limits. In contrast with blood pressure, which appears to be tightly controlled, extracellular fluid volume varies significantly, even in normal individuals, when dietary NaCl intake changes. Cardiac, liver, or renal disease can perturb the relationship between NaCl intake and extracellular fluid volume and lead to symptomatic edema. All major classes of diuretic drugs in use today were developed between 1950 and 1970. These drugs were developed empirically, without knowledge of specific ion transport pathways, but experimental work during the past 15 years has shown that each major class of diuretic inhibits a specific ion transport protein in the kidney. These transport proteins have been characterized physiologically and the mechanisms by which each diuretic drug inhibits ion transport have been defined. Antibodies directed against these transport proteins have localized ion transport pathways to specific cell types along the nephron. Most recently, isoforms of each class of diuretic-sensitive Na transport pathway have been cloned. Ongoing experimental work is aimed at exploring relationships between families of transporters, determining the structural prerequisites for ion transport, and studying molecular mechanisms of transport regulation. Treatment of edema with diuretics is often straightforward, but can lead to adaptive changes in nephron structure and function. These adaptations can limit the effectiveness of diuretic drugs; maneuvers aimed at blocking these processes can be effective approaches to patients who are resistant to diuretic drugs.

Transfusion of carbonic anhydrase-replete erythrocytes fails to correct the acidification defect in the syndrome of osteopetrosis, renal tubular acidosis, and cerebral calcification (carbonic anhydrase-II deficiency)

We explored the effects of transfusion of carbonic anhydrase II (CA-II)-replete erythrocytes on systemic pH, serum electrolytes, and urinary acidification of a patient with CA-II deficiency. Pretransfusion studies documented hyperchloremic acidosis, increased urinary pH with decreased titratable acidity, and profound CA-II deficiency in erythrocytes. During transfusion, CA-II in circulating erythrocytes increased to above the half-normal levels seen in asymptomatic heterozygote carriers of CA-II deficiency. However, no significant change occurred in venous, arterial or urinary pH, serum electrolytes, and urinary acid excretion during the transfusion or during the subsequent 60 hr of observation. These studies argue that the renal acidification defect in CA-II deficiency results from deficiency of CA-II in the renal parenchyma, and is not secondary to deficiency of CA-II in erythrocytes. Bone marrow transplantation is not a promising approach to correct the renal manifestations of CA-II deficiency.

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.