[Regulation of crystalline lens aldose reductase activity. Nonhyperbolic oxidation kinetics of NADPH by glucose]

A homogeneous aldose reductase was isolated from bovine eye lens tissue by using affinity chromatography on blue agarose. A kinetic analysis of the initial rates of NADPH oxidation at 0.5-100 mM glucose and at 1.2-10 microM NADPH was carried out. The Line-weaver-Burk plots for glucose concentration were nonlinear at fixed concentrations of NADPH and linear at fixed concentrations of glucose. It was shown that the experimental plots reflect the mechanisms, in which substrate regulation of enzyme activity is effectuated by glucose binding to the regulatory site or is due to the shift of the equilibrium between the isomeric forms of aldose reductase.

Calcium-induced opacification and proteolysis in the intact rat lens

When intact rat lenses were incubated in artificial aqueous humor in the presence of 1 mM calcium and a sulfhydryl reagent p-chloromercuriphenyl sulfonate (pCMPS) a visible annular opacity developed within 4 hours. Combined photographic and ion-sensitive microelectrode investigations of the lenses demonstrated that the subsequent linear increase in opacification was accompanied by an increase in internal free calcium. Opacities were not observed in lenses incubated in the absence of either pCMPS or calcium. Gel electrophoresis of the soluble and urea-soluble fractions from lenses exposed to 1 mM calcium for periods of up to 14 hours showed no evidence for crystallin degradation and only minor proteolysis of cytoskeletal proteins. When lenses were incubated under identical conditions, but with 5 mM calcium, the degree of opacification increased up to approximately 8 hours and then remained constant. A progressive loss in cytoskeletal proteins was observed which correlated with a further increase in free calcium such that by 14 hours of incubation, when the internal calcium approached 1 mM, most of the spectrin and vimentin present in the cortex of the lens had disappeared. An unidentified 110-kilodalton protein also disappeared from lenses incubated in 5 mM calcium. These results indicate that proteolysis by calcium-dependent enzymes such as calpain may play a significant role in cytoskeletal regulation and metabolism in the lens. A role for cytoskeleton/membrane/crystallin interaction in calcium-induced opacification is discussed.

Studies on aldose reductase inhibitors from medicinal plant of "sinfito," Potentilla candicans, and further synthesis of their related compounds

For several years we have screened natural products having aldose reductase (AR) inhibitory activity. 3,3',4-Tri-O-methylellagic acid 4'-sulfate potassium salt (2) was isolated from a Mexican herb "Sinfito" (Potentilla candicans) as a potent AR inhibitory active constituent. 2 was more potent (IC50 = 8.0 x 10(-8)M) than ellagic acid, which is one of the natural inhibitors of AR. So we examined the synthesis of ellagic acid derivatives and found that the sulfate group is one of the important function.

Calpain II in two in vivo models of sugar cataract

Cataracts were produced in rat lenses by either feeding a diet containing 50% galactose or by inducing diabetic condition by intravenous injection of streptozotocin. Proteolysis of crystallins, protease activity of calpain II enzyme (EC 3.4.22.17), and presence of calpain molecule (antigen) were determined at four cataract stages--I, cortical vacuoles, II, vacuoles plus hazy cortex, III, nuclear cataract, and IV, mature cataracts. Calpain activity was normal or moderately elevated at early stages of galactose and diabetic cataracts. Later stages III and IV showed proteolysis of lens crystallins, increased proportion of insoluble proteins, loss of calpain enzyme activity and calpain molecule from the soluble fraction, and reduced amounts of calpain associated with insoluble pellet. In galactose cataract, the largest increase in lens calcium were found when proteolysis was present. These results provide evidence for calpain-induced proteolysis of lens crystallins in two in vivo models of sugar cataracts in rodents.

Alteration of sodium, potassium-adenosine triphosphatase activity in rabbit ciliary processes by cyclic adenosine monophosphate-dependent protein kinase

The response of sodium, potassium-adenosine triphosphatase (Na,K-ATPase) to cyclic adenosine monophosphate (cAMP)-dependent protein kinase was examined in membranes obtained from rabbit iris-ciliary body. In the presence of the protein kinase together with 10(-5) M cAMP, Na,K-ATPase activity was reduced. No change in Na,K-ATPase activity was detected in response to the protein kinase without added cAMP. Likewise cAMP alone did not alter Na,K-ATPase activity. Reduction of Na,K-ATPase activity was also observed in the presence of the cAMP-dependent protein kinase catalytic subunit. The response of the enzyme to the kinase catalytic subunit was also examined in membranes obtained from rabbit ciliary processes. In the presence of 8 micrograms/ml of the catalytic subunit, ciliary process Na,K-ATPase activity was reduced by more than 50%. To examine whether other ATPases were suppressed by the protein kinase, calcium-stimulated ATPase activity was examined; its activity was stimulated by the catalytic subunit. To test whether the response of the ciliary process Na,K-ATPase is unique, experiments were also performed using membrane preparations from rabbit lens epithelium or rabbit kidney; the catalytic subunit significantly reduced the activity of Na,K-ATPase from the kidney but not the lens. These Na,K-ATPase studies suggest that in the iris-ciliary body, cAMP may alter sodium pump activity. In parallel 86Rb uptake studies, we observed that ouabain-inhibitable potassium uptake by intact pieces of iris-ciliary body was reduced by exogenous dibutryl cAMP or by forskolin.

(Pyrimidinyloxy)acetic acids and pyrimidineacetic acids as a novel class of aldose reductase inhibitors

Pyrimidineacetic acids and (pyrimidinyloxy)acetic acids were synthesized by alkylation, with methyl bromoacetate or tert-butyl bromoacetate as alkylating agents. Alkylation reaction at the nitrogen or oxygen atom for different substrates was found to be solvent dependent. N-Alkylation was favored in ethereal solvent, e.g., tetrahydrofuran and dimethoxyethane, whereas O-alkylation was predominant in dimethylformamide. These compounds were tested in vitro to determine their ability to inhibit bovine lens aldose reductase. Selected compounds were assayed in vivo, in a 4-day galactose-fed rat model. The decrease in galactitol from the control was determined in lens, nerve, and diaphragm. Several of the 6-oxopyrimidine-1-acetic acids and (pyrimidinyl-4-oxy)acetic acids were found to be potent inhibitors of bovine lens aldose reductase. A study was also undertaken to determine in vitro the transport behavior of selected compounds in the isolated rat sciatic nerve. A discussion of the structure-activity relationship of this class of compounds with reference to their intrinsic biochemical activity is reported. It is concluded, in general, that ability of a compound to penetrate the tissue membrane plays an important role in the genesis of in vivo lens aldose reductase (LAR) inhibitory activity.

Inhibition of aldehyde reductase by aldose reductase inhibitors

A broad group of structurally diverse aldose reductase inhibitors including flavonoids, carboxylic acids and hydantoins, have been examined for their ability to inhibit rat kidney aldehyde reductase (EC 1.1.1.19, EC 1.1.1.20) versus rat lens aldose reductase (EC 1.1.1.21). All aldose reductase inhibitors examined inhibited aldehyde reductase to some extent both in the reductive reaction as determined with glyceraldehyde as substrate and NADPH as coenzyme, and in the oxidative reaction where L-gulonic acid was oxidized to D-glucuronic acid in the presence of NADP+. Of the inhibitors examined, 2,7-difluorospirofluorene-9,5'-imidazolidine-2',4'-dion e (Al1576) was the most potent inhibitor requiring only concentrations in the 10(-8) M range to inhibit 50% of the in vitro activity of rat kidney aldehyde reductase (IC50 value), whereas 3-dioxo-1-H-benz[de]isoquinoline-2(3H)-acetic acid (alrestatin) was the least potent inhibitor requiring concentrations in the 10(-5) M range. Both the reductive and oxidative steps appeared equally inhibited by these aldose reductases inhibitors. Moreover, all compounds appeared to inhibit either crude or highly purified rat kidney aldehyde reductase to essentially the same extent. Marked differences in the selectivity of these inhibitors, expressed as the ratio of IC50 values for rat kidney aldehyde reductase versus rat lens aldose reductase with glyceraldehyde as substrate, were observed with selectivity for aldose reductase ranging from ca. 2-fold for Al1576 to 119-fold for 3-(4-bromo-2-fluorobenzyl-4-oxo-3-phthalazine-1-ylacetic acid (Ponalrestat). Kinetic and competition studies suggest that these inhibitors interact with aldehyde reductase at a common site that is not identical to either the substrate or nucleotide binding site. These results suggest that the inhibitor binding sites of rat kidney aldehyde reductase and aldose reductase contain several common characteristics.

Aldose reductase mRNA is an epithelial cell-specific gene transcript in both normal and cataractous rat lens

Aldose reductase (AR) is implicated in the development of sugar cataracts by its reduction of galactose or glucose to polyols. The authors' recent work suggested that AR mRNA is found to be expressed in high concentrations in rat-lens epithelial cells after exposure of the animal to a diet containing 50% galactose. They localized the AR mRNA in the lens cells by in situ hybridization with a previously described AR clone. The data establish that AR mRNA is apparently an epithelial and not a fiber-cell gene transcript. It accumulates in the epithelial cell, then it is carried into the newly differentiated fiber cell, and finally it concentrates in the posterior region of the matured fiber cell. The AR mRNA is found in all of the anterior epithelial cells including the equatorial and central epithelia. It is present at highest concentrations in the elongating epithelial cells, and it is distributed equally throughout the secondary fiber cells at the bow, with no indication of a preferential buildup of AR mRNA in any of the nucleated fiber cells in the cortex. This differs from what the authors reported to occur with MP26 mRNA, a fiber cell-specific gene transcript. They found that MP26 mRNA was absent from the epithelial cells but was preferentially found in the secondary fiber cells. Present data suggest that the increase in AR mRNA concentration observed to occur in cataractous lenses is a result of epithelial cell proliferation, where every cell appears to be competent in expressing AR mRNA. The results of this research imply that AR mRNA is a lens epithelial cell-specific gene transcript in both normal and cataractous lenses.

Enhanced depletion of lens reduced glutathione Adriamycin in riboflavin-deficient rats

The anticancer drug Adriamycin has photosensitizing properties which potentially may be detrimental to lens tissue. Since reduced glutathione (GSH) serves to protect lens from photo-oxidative stress and dietary riboflavin is required by glutathione reductase to regenerate GSH, we investigated whether Adriamycin intensifies the depletion of GSH levels in rat lens during dietary riboflavin deficiency. Three-week-old rats were divided into two groups. One group was fed a diet deficient in riboflavin (less than 1 ppm) and the other group was pair-fed a control diet containing adequate riboflavin (8.5 ppm). After 6-12 weeks of dietary treatment, half the animals in each dietary group received Adriamycin (8 mg/kg/day) intraperitoneally for 3 days. After killing the rats, lenses were removed, and GSH content and glutathione reductase activity were measured in freshly prepared homogenates. To determine the extent of systemic oxidative stress and the degree of riboflavin deficiency, glucose-6-phosphate dehydrogenase and glutathione reductase activities, respectively, were measured in erythrocytes. In lens of rats fed the riboflavin-sufficient diet, treatment with Adriamycin did not diminish GSH content or alter glutathione reductase activity. In confirmation of reports by others, lenses of animals fed the riboflavin-deficient diet had diminished GSH levels, lower basal glutathione reductase activity, and elevated glutathione reductase activity coefficients compared to those of animals pair-fed the control diet. The present study shows that in riboflavin-deficient rats, Adriamycin exacerbated the depletion of GSH but did not reduce further glutathione reductase activity. The implications of these findings are that nutritional deficiencies, in particular riboflavin deprivation, may pose a potential risk to lenticular tissue following Adriamycin treatment.

In vitro expression of rat lens aldose reductase in Escherichia coli

Aldose reductase (alditol:NADP+ oxidoreductase, EC 1.1.1.21), an enzyme that converts glucose to sorbitol, the first step of the polyol pathway, has been implicated in secondary complications of diabetes, such as cataracts, retinopathy, neuropathy, and nephropathy. Aldose reductase inhibitors have been observed to prevent or delay the onset of these complications; however, more potent and specific inhibitors are needed. Development of new inhibitors necessitates a better understanding of the molecular structure of this protein. To elucidate the structure-function relationships of aldose reductase and to develop methods of regulating this enzyme, large and homogeneous quantities of rat lens aldose reductase have been expressed in bacterial cells. A construction of the complete coding sequence and 3' untranslated region for rat lens aldose reductase was assembled in the expression vector pKK233-2 (Pharmacia). This construction expresses an active enzyme that has been purified and demonstrates kinetic, immunological, and inhibitory properties similar to rat lens aldose reductase.

Purification and characterization of rho-crystallin from Japanese common bullfrog lens

In a previous paper, we reported that the partial amino acid sequence (225 residues) from the COOH terminus of rho-crystallin from European common frog lens shows 77% similarity to that of prostaglandin (PG) F synthetase, an aldo-keto reductase, from bovine lung (Watanabe, K., Fujii, Y., Nakayama, K., Ohkubo, H., Kuramitsu, S., Kagamiyama, H., Nakanishi, S., and Hayaishi, O. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 11-15). Here rho-crystallin was purified to apparent homogeneity from the eye lens of the Japanese common bullfrog (Rana catesbeiana) by four sequential chromatographies using Sephadex G-100, Red Sepharose, and dual Mono S. Two types of rho-crystallin, RHO-I and RHO-II, named according to their elution order from a Mono S column, are essentially identical in terms of immunochemical properties, amino acid composition, and partial amino acid sequence. But the NH2-terminal Thr of RHO-I is blocked with an acyl group, while that of RHO-II is free. Both crystallins as well as PGF synthetase are monomeric proteins with a molecular weight of about 35,000 and they have the ability to bind NADPH with a stoichiometry of 0.75 mol of cofactor/mol of protein. Although rho-crystallin does not cross-react with antibody against PGF synthetase, the NH2-terminal amino acid sequence (107 residues) of rho-crystallin shows 77% similarity to that of the enzyme. However, PGD2, PGE2, 9,10-phenanthrenequinone, p-nitrobenzaldehyde, DL-glyceraldehyde, D-glucuronic acid, D-glucose, D-xylose, menadione, p-nitroacetophenone, dihydroxyacetone, succinic semialdehyde, phenylglyoxal, and testosterone were not substrates for these crystallins. PGH2 9,11-endoperoxide reductase activities of RHO-I and RHO-II were 1.3 and 1.0 milliunits/mg of protein, respectively, which are only about 2% of that of bovine lung PGF synthetase. These results indicate that the rho-crystallins RHO-I and RHO-II belong to a group of aldo-keto reductases based on primary structure, molecular properties, and NADPH-binding ability, but show only low PGH2 9,11-endoperoxide reductase activity.

Management of aldose reductase mRNA abundance in rat lens undergoing reversal of galactose induced cataracts. A model for gene response to changes in the environment

Aldose reductase (AR) mRNA concentration in rat lens was quantitated by hybridization of a RNA transcript from a previously described AR cDNA clone to mRNA found in epithelial and cortical cytosols. This was done on normal rat lens and on lens initially made cataractous by feeding of a diet of Purina Chow containing 50% galactose, followed by reversal of the cataracts due to the removal of the galactose from the diet. Recent data from this laboratory has shown that AR mRNA was increased in lens epithelial cells upon administration of galactose; while in the cortex it was reduced to insignificant levels when fiber cell damage became extensive by day 20 on galactose. Present data reveals that, upon removal of galactose from the diet, the lens epithelial AR mRNA was gradually reduced from the high levels found at day 20 of galactose feeding to low levels by day 30 of reversal. On the other hand, the cortex exhibited an initial sudden increase in AR mRNA at days 1 to 6 of reversal and by day 30 it was reduced to levels below those found in the untreated lens. DNA content in the epithelium also began to decrease to normal levels by day 16 following reversal of the cataracts. The data demonstrate that the concentration of AR mRNA in lens of reversed cataracts appears to faithfully reflect the loss of epithelial cellular need for AR mRNA in favor of enhanced differentiation of epithelial cells to secondary fiber cells.

Cleavage of A-CAM by endogenous proteinases in cultured lens cells and in developing chick embryos

We describe two truncated forms of A-CAM (N-cadherin) and present evidence suggesting that both forms are proteolytically derived from the intact A-CAM molecule. The first is a membrane-bound fragment of A-CAM displaying an apparent molecular weight of 78 kDa. This polypeptide, containing the C-terminal portion of the protein, may be generated in cultured chicken lens cells, either by a short treatment with trypsin-EGTA, or by endogenous proteinase(s) during incubation in low Ca2+ medium. Immunofluorescent labeling of normal and EGTA-treated cells indicated that the 78-kDa fragment is uniformly distributed over the cell surface. Moreover, staining of developing chick embryos with pairs of antibodies which distinguish the 78-kDa fragment from intact A-CAM indicated that, at early stages of sclerotome dissociation in developing somites, a truncated derivative of the molecule is generated. The second truncated form of A-CAM is a 97-kDa polypeptide which is constitutively released by cultured lens cells into the culture medium in the presence of normal medium. We present evidence that the 97-kDa molecule is proteolytically derived from A-CAM by the action of an endogenous proteinase. We discuss possible mechanisms leading to the formation of these two truncated derivatives and their possible involvement in the physiological modulation of A-CAM-mediated interactions.

Activities of ascorbate free radical reductase and H2O2-dependent NADH oxidation in senile cataractous human lenses

Changes in activities of ascorbate free radical (AFR) reductase (NADH:AFR oxidoreductase) and H2O2-dependent NADH oxidation were correlated with levels of insoluble protein in senile cataractous human lenses. The H2O2-dependent NADH oxidation activity was measured to reflect the content of free glutathione. AFR reductase activities in all the cataractous lenses assayed here tended to decrease with increase of insoluble protein contents. A similar tendency in the relationship between lens protein aggregation and H2O2-dependent NADH oxidation activities, i.e. free glutathione contents was recognized in the lenses with pale yellow, yellow or dark yellow nucleus. However, for the highest levels of insoluble protein, some of the brunescent cataractous lenses exhibited very high activities of H2O2-dependent NADH oxidation, and some brunescent lenses had very low activities. From the above results, it is suggested that lens protein aggregates in the brunescent and non-brunescent cataractous lenses may be formed through significantly different oxidation processes, respectively. The possible mechanisms such as free radical reaction and disulfide bond formation are discussed.

Photo-oxidative damage to lenticular GAPDH and its relationship to aldehyde metabolism

These studies show that GAPDH from the calf lens and human lens epithelium undergoes size and charge modifications in the presence of active oxygen species generated from methylene blue and light. The histidine content of the enzyme was reduced by one third. No evidence of increased proteolysis was observed suggesting a nonenzymatic cleavage of enzyme by the photo-oxidants.

NADPH-dependent reductases of the dog lens

The presence of several NADPH-dependent reductases has been observed in the dog lens. Applying the purification procedures of gel filtration, affinity chromatography and chromatofocusing to dog lens homogenates resulted in the purification of aldose reductase. This enzyme appeared similar to dog kidney aldose reductase in molecular weight, isoelectric point, kinetic properties, and susceptibility to inhibition by aldose reductase inhibitors. Evidence for the presence of trace amounts of aldehyde reductase in the dog lens was also observed, although this enzyme is not present in sufficient quantities for isolation and characterization. The presence of a labile third enzyme that is immunologically distinct from either aldose reductase or aldehyde reductase was also detected. This enzyme utilizes only glyceraldehyde as substrate and is not inhibited by aldose reductase inhibitors.

Increase in aldose reductase mRNA in dog lens epithelial cells under hypertonic conditions

Aldose reductase (AR) mRNA levels increase when dog lens epithelial cells are exposed to hypertonic conditions. Hybridization of mRNA to an AR cDNA, using Northern and slot blots, showed that AR mRNA is elevated at least fourfold when primary dog lens epithelial cells are grown in media (300 mosmol kg-1) supplemented with 150 mM NaCl (600 mosmol kg-1 final). A time course showed an increase in AR mRNA of approximately twofold by 24 hr with a maximum increase of between four- and eightfold by 48 hr. AR mRNA remained elevated for the duration of the experiment, 8 days. The addition of Tolrestat, an inhibitor of aldose reductase, had no effect on the increased level of AR mRNA in these hypertonically stressed cells. Cells grown in media supplemented with 250 mM sorbitol also showed a substantial increase in AR mRNA. These data indicate, as in other cell types, the lens, a target tissue of diabetes, responds to hypersomotic stress with an induction of AR expression and suggests that AR may play a role in intracellular osmotic regulation.

[Detection of myeloperoxidase in eye tissues of man]

Myeloperoxidase (MPO) is an enzyme which usually detected in leukocytes of peripheral blood. Its principal function is to produce bactericidal hypochlorite-ions in reaction of hydrogen peroxide with chlor ions. The present work deals with testing myeloperoxidase activity in different eye tissues. Evolution of MPO activity has been tested by means of dianisidine reaction and immunochemical identification in extracts. High activity of MPO was found for retina and lens of healthy men and elderly people with lens opalescence. It should be noted that cellular body, cornea, aqueous humor, vitreous humor do not manifest any noticeable activity. Iris has low activity.

Human and murine urokinase cDNAs linked to the murine alpha A-crystallin promoter exhibit lens and non-lens expression in transgenic mice

cDNAs encoding either the human or the murine urokinase-type plasminogen activator (uPA) were fused downstream from the promoter-enhancer element of the murine gene encoding alpha A-crystallin, a protein found exclusively in the ocular lens. The DNAs were microinjected into fertilized mouse eggs as linear fragments free of bacterial sequences, and for each construct one line of transgenic mice was generated. In both lines transgenic uPA activity was detected in the ocular lens, in agreement with previous results reported on transgenic mice bearing genes fused to the same regulatory region. Unexpectedly however relatively high levels of this activity were found also in the retina, and furthermore, human uPA activity was found also in different parts of the brain and in the bone marrow, and to a lesser extent in the spleen, thymus and optic nerve. Transgenic uPA transcript was found in the lens, retina, brain and thymus of mice carrying the murine cDNA. Such a pattern of expression was different from that exhibited by the endogenous murine uPA gene and, excluding the lens, it appeared to be conferred by the cDNAs. The putative regulation by uPA cDNAs is suggested to be mediated through an internal enhancer-like element functioning in combination with the alpha A-crystallin promoter in a fashion independent of the specific nature of the promoter.

In vitro study of aldose reductase inhibitor concentrations in the lens and inhibitory effect on sugar alcohol accumulation

Rat lenses were cultured in a medium containing either 30mM xylose or 30mM glucose. Inhibitory effects of two aldose reductase inhibitors, AD-5467 and CT-112, on the accumulation of sugar alcohols and the concentrations of the drugs in the lens were measured. Inhibitory effect of lens opacification by the drugs were also observed. Lens opacification was apparently inhibited in a medium containing each of the drugs. Inhibition rates for sugar alcohol accumulation by AD-5467 and CT-112 were unrelated to the kinds of added sugars. 50% inhibition-concentration in the medium was 0.82-1.02 x 10(-7) M for AD-5467 and 3.78-5.92 x 10(-7) M for CT-112, and the resulting concentration of drug in the lens was 2.33-3.08 nmole/g lens for AD-5467 vs. 21.19-24.63 nmole/g lens for CT-112. The results above indicated that the concentration of CT-112 should be 4.6-5.8 times of that of AD-5467 in the medium to attain the same inhibitory effect on sugar alcohol accumulation, and 8.0-9.1 times more CT-112 is needed in the lens. A linear correlation was found on a bilogarithmic graph of concentrations in the medium and in the lens for both drugs. This correlation is an empirical adsorption isotherm shown by Freundrich. So this suggested that both drugs were adsorbed into the lens. The rate of adsorption varied with the drugs. Even when the drug concentrations in the medium were the same, the concentration of CT-112 in the lens was 2-4 times higher than that of AD-5467.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequence analysis of bovine lens aldose reductase

The covalent structure of bovine lens aldose reductase (alditol-NADP+ oxidoreductase, EC 1.1.1.21) was determined by sequence analysis of peptides generated by specific and chemical cleavage of the homogeneous apoenzyme. Peptides, purified by reverse-phase high performance liquid chromatography were subjected to compositional analysis and sequencing by gas-phase automated Edman degradation. Aldose reductase was found to contain 315 amino acid residues. The enzyme is blocked at the amino terminus, and mass spectrometry was employed to identify the blocking acetyl group and to sequence the amino-terminal tryptic peptide. The aldose reductase was shown to contain no carbohydrate despite the fact that the enzyme contains the consensus sequence -Asn-Lys-Thr- for N-linked glycosylation. Comparative sequence analysis and application of algorithms for prediction of secondary structure and nucleotide binding domains are consistent with the view that aldose reductase is a double-domain protein with a beta-alpha-beta secondary structural organization. The NADPH binding site appears to be associated with the amino-terminal half of the enzyme. Modeling studies based on the tertiary structures of dihydrofolate and glutathione reductases indicate that the NADPH binding site begins at Lys-11 and continues with a beta-alpha-beta fold characteristic of nucleotide binding proteins.

[cAMP-independent protein kinase from amphibian lens: identification, organ distribution and substrates of phosphorylation]

Eye lens extracts of the frog Rana temporaria contain a cAMP-independent protein kinase which is quantitatively adsorbed on immobilized RNA at physiological salt concentrations. The enzyme activity is maximal in the lenticular cortex, medium in the epithelium and minimal in the lens nuclei. Crude preparations of RNA-binding protein kinase from the epithelium, cortex and nuclei of the eye lens were prepared by affinity chromatography on poly(U)-Sepharose. It was found that these preparations contain no active forms of phosphatases, ATPases or proteases which may interfere with the results of phosphorylation experiments on exogenous and endogenous substrates. The protein kinase under study catalyzes the binding of phosphate groups to threonine and serine residues in casein molecules, does not phosphorylate histones and utilizes GTP alongside with ATP as phosphate donors. Heparin and RNA used at low concentrations inhibit the protein kinase activity. The data obtained allow the identification of lenticular RNA-binding protein kinase(s) as a casein kinase type II. It was shown that incubation of RNA-binding proteins from epithelium and lenticular cortex with [gamma-32P]ATP results in the label incorporation into six to seven polypeptide chains with Mr of 27-130 kDa. Poly(U) and heparin inhibit the self-phosphorylation reaction, cAMP has no stimulating effect on this process, while Ca2+ ions inhibit the self-phosphorylation of RNA-binding proteins.

Enolase in the avian and turtle lens

Enolase is a dimeric enzyme of molecular weight of 100,000 daltons, which plays an important role in the glycolytic cycle. The aim of this study was to characterize the enzyme of the chicken lens epithelium and to compare its distribution in different regions of the chicken, duck and turtle lens. Enolase of the chicken lens epithelium was found to be an enzymatically active dimeric protein of molecular weight 100,000 daltons and representing alpha-enolase. It is a major component of the epithelium comprising 4%, 12% and 46% of the water-soluble protein of chicken, duck and turtle epithelium respectively. Enolase is found in trace amount in the fiber cells of the chicken and duck, but is retained in much greater concentration in the turtle fiber mass as a predominantly inactive enzyme.

[A theoretical study on the regulation mechanism of Ca2+ concentration in lens related to the Ca2+ dependence of Ca2(+)-ATPase activity]

A four-state model for the elementary process of the Ca2+ active transport system in lens was proposed, and based upon this model the Ca2+ dependence of Ca2(+)-ATPase activity was analyzed. The results indicated that the Ca2(+)-ATPase activity reaches a peak at approximately pCa 4-5, and decreases at higher and lower Ca2+ concentrations. In the range of pCa 5-6, ATPase activity increases with Ca2+ concentration. Therefore, the model accounts for the kinetic properties of Ca2(+)-ATPase is a qualitative way. Moreover, it is proved that the Ca2+ dependence of Ca2(+)-ATPase activity is theoretically based upon the changes in Ca2+ affinity of Ca2+ binding sites and the Ca2+ dependence of Ca2(+)-ATPase activity is possibly involved in the regulation of intracellular Ca2+ concentration in vivo as a switching mechanism.

Serine hydroxymethyltransferase: evidence for its presence in human, monkey and rat lenses

Serine hydroxymethyltransferase (SHMT) is present in cultured rat, monkey and human lenses as shown by 15N-serine or 15N-glycine labeling studies. Following incubation with 15N-serine, the percent enrichment of 15N in glycine increases with time, and vice versa, demonstrating the presence of the enzyme, and the reversibility of the reaction in intact cultured lenses. Similar patterns of 15N enrichment were found in all three species, but lenses from young rats showed a higher percent enrichment than lenses obtained from older animals. Label from 15N-serine or 15N-glycine was also incorporated into a number of other amino acids, including aspartate, alanine, glutamate and proline. Conclusive evidence for the presence of SHMT in rat lens homogenates has been obtained by direct enzyme assay. The specific activity of rat lens SHMT was age dependent; approximately 2.4 units per mg protein in day old rats, declining to about 0.15 units per mg in adult animals. The higher specific activity observed in younger animals is consistent with the 15N labeling results obtained with cultured lenses. Lens SHMT has been partially characterized. In the presence of excess tetrahydrofolate the assay was essentially linear with increasing time. With serine as the substrate, the enzyme requires tetrahydrofolate for activity, the pH optimum is between pH 7.5 and 8.3, the Km for serine is about 0.25 mM, and the enzyme is inhibited by cycloserine. In conclusion, this study demonstrates the existence of SHMT in rat, monkey and human lenses. Rat lens specific activity has been shown to decrease with increasing age, and the enzyme has been partially characterized.(ABSTRACT TRUNCATED AT 250 WORDS)