Cation pump activity and membrane permeability in human senile cataractous lenses

Levels of Trace Elements in the Lens, Aqueous Humour, and Plasma of Cataractous Patients-A Narrative Review

Cataracts are one of the most common causes of effective vision loss. Although most cases of cataracts are related to the ageing process, identifying modifiable risk factors can prevent their onset or progression. Many studies have suggested that micro and macroelement levels, not only in blood serum but also in the lens and aqueous humour, may affect the risk of the occurrence and severity of cataracts. This systematic review aims to summarise existing scientific reports concerning the importance of trace elements in cataractogenesis. Many authors have pointed out elevated or decreased levels of particular elements in distinct ocular compartments. However, it is not known if these alterations directly affect the increased risk of cataract occurrence. Further studies are needed to show whether changes in the levels of these elements are correlated with cataract severity and type. Such information would be useful for determining specific recommendations for micronutrient supplementation in preventing cataractogenesis.

Differential membrane redistribution of P2X receptor isoforms in response to osmotic and hyperglycemic stress in the rat lens

P2X(1, 2, 3, 4, 6 and 7) are all expressed in a differentiation-dependent manner in the rat lens. However, in the lens outer cortex the subcellular distribution of all P2X isoforms is predominantly associated with a pool of receptors located in cytoplasmic vesicles. Here we investigate whether osmotic and hyperglycemic stress can alter the subcellular distribution of this cytoplasmic pool of P2X receptors. We show that in a discrete zone of the deeper outer cortex an isoform and stimulus-specific shift in the subcellular distribution of P2X receptors occurs from the cytoplasm to defined membrane domains. In response to hypertonic stress P2X(1) and P2X(4) isoforms became more closely associated with the broad sides of fiber cells, while under hypotonic conditions P2X(4) and P2X(6) isoforms associate with the narrow side membranes. No such changes in subcellular distribution were observed for P2X(2,3 and 7) isoforms. Lens cultured in 50 mM glucose exhibited cell swelling in this zone but only P2X(4) associated with narrow side membranes. Our results indicate P2X receptors can be differentially recruited to specific membrane domains of lens fiber cells by osmotic and hyperglycemic stress. Furthermore they suggest the involvement of specific P2X isoforms in the regulation of fiber cell volume and the initiation of diabetic cataract.

Oxidation-induced changes in human lens epithelial cells 2. Mitochondria and the generation of reactive oxygen species

The relationships among reactive oxygen species (ROS) generation, lipid compositional changes, antioxidant power, and mitochondrial membrane potential were determined in a human lens epithelial cell line, HLE-B3. Cells grown in a hyperoxic atmosphere grew linearly for about 3 days, and then progressively died. Total antioxidant power and ROS generation increased by 50 and 43%, respectively, in cells grown in a hyperoxic atmosphere compared to those cultured in a normoxic atmosphere. By specifically uncoupling the mitochondrial proton gradient, we determined that the mitochondria are most likely the major source of ROS generation. ROS generation correlated inversely with mitochondrial membrane potential and the amount of cardiolipin, factors likely to contribute to loss of cell viability. Our results support the idea that hyperoxic damage to HLE-B3 cells derives from enhanced generation of ROS from the mitochondrial electron transport chain resulting in the oxidation of cardiolipin. With extended hyperoxic insult, the oxidants overwhelm the antioxidant defense system and eventually cell death ensues.

Synergism between phospholipase D2 and sorbitol accumulation in diabetic cataract formation through modulation of Na,K-ATPase activity and osmotic stress

Phospholipase D (PLD), a highly regulated enzyme that generates the second messenger phosphatidic acid, functions in signal transduction, membrane trafficking and cytoskeletal reorganization. PLD is thought to be involved in the pathogenesis of diabetic complications by activating PKC. Since PKC and PLD are present in the lens we sought to determine if PLD plays a role in diabetic cataract development. We developed transgenic mice that overexpress PLD2, one of the two mammalian isoforms of PLD. These mice developed congenital nuclear cataracts, but not diabetic cataracts. Histological analysis revealed vacuole formation in the fiber cells, mediated potentially by the substantially increased Na,K-ATPase activity. In the presence of the aldose reductase overexpressing transgene that increases lens osmotic pressure, these double transgenic mice developed more severe congenital cataract and became susceptible to develop diabetic cataract. Together, these data suggest that increased PLD2 activity in the lens under hyperglycemic condition might impair its osmoregulatory mechanism and reduce its ability to cope with the osmotic stress triggered by sorbitol accumulation.

Plasma membrane Ca2+-ATPase expression in the human lens

The focus of the study was to characterize plasma membrane calcium-ATPase pump (PMCA) isoform expression in the human lens and cultured lens epithelial cells as a basis for future studies of calcium homeostasis in the lens. Proteins and mRNA expression were analysed using Western Immunoblotting and reverse transcription polymerase chain reaction (RT-PCR), respectively. Clear human lenses from the Kentucky Lions Eye Bank and an immortalized human lens epithelial cell line (HLE B-3) were used. RT-PCR products of PMCA1, PMCA2, and PMCA4 primers were detected at 429, 557, and 849bp, respectively. All these products were identified as PMCA isoforms by sequence analysis. Protein bands at approximately 130, 115, and 135kDa were detected by Western blot analysis for PMCA1, PMCA2 and PMCA4, respectively. PMCA3 was not detected at protein or mRNA level in any human lens sample or cell culture, but was detected in the rat brain cortex used as a control. Several bands with lower molecular weights, especially for PMCA2, were detected in the epithelial samples and probably represent break down products of PMCA2. No PMCA proteins or breakdown products were detected in the nuclear or cortical fractions from human lenses. PMCA1, 2, and 4 proteins and mRNAs are expressed in human lens epithelium and cultured epithelial cells; PMCA3 is not. PMCA was not detected at all in the lens fibre cells. The calcium pump must be selectively processed, independent of other membrane proteins such as the Na-K-ATPase pumps, because the distribution of the Na-K-ATPase pump is asymmetrical in the epithelium and present throughout the lens whereas the calcium pumps are not. The findings of this study provide a basis for further studies to examine the role and modulation of PMCA isoforms in calcium homeostasis and in the development of cataract.

Expression, regulation and function of Na,K-ATPase in the lens

Na,K-ATPase is responsible for maintaining the correct concentrations of sodium and potassium in lens cells. Na,K-ATPase activity is different in the two cell types that make up the lens, epithelial cells and fibers; specific activity in the epithelium is higher than in fibers. In some parts of the fiber mass Na,K-ATPase activity is barely detectable. There is a large body of evidence that suggests Na,K-ATPase-mediated ion transport by the epithelium contributes significantly to the regulation of ionic composition in the entire lens. In some species different Na,K-ATPase isoforms are present in epithelium and fibers but in general, fibers and epithelium express a similar amount of Na,K-ATPase protein. Turnover of Na,K-ATPase by protein synthesis may contribute to preservation of high Na,K-ATPase activity in the epithelium. In ageing lens fibers, oxidation, and glycation may decrease Na,K-ATPase activity. Na,K-ATPase activity in lens fibers and epithelium also may be subject to regulation as the result of protein tyrosine phosphorylation. Moreover, activation of G protein-coupled receptors by agonists such as endothelin-1 elicits changes of Na,K-ATPase activity. The asymmetrical distribution of Na,K-ATPase activity in the epithelium and fibers may contribute to ionic currents that flow in and around the lens. Studies on human cataract and experimental cataract in animals reveal changes of Na,K-ATPase activity but no clear pattern is evident. However, there is a convincing link between abnormal elevation of lens sodium and the opacification of the lens cortex that occurs in age-related human cataract.

Evidence for distinct cholesterol domains in fiber cell membranes from cataractous human lenses

Previous studies in our laboratory have provided direct evidence for the existence of distinct cholesterol domains within the plasma membranes of human ocular lens fiber cells. The fiber cell plasma membrane is unique in that it contains unusually high concentrations of cholesterol, with cholesterol to phospholipid (C/P) mole ratios ranging from 1 to 4. Since membrane cholesterol content is disturbed in the development of cataracts, it was hypothesized that perturbation of cholesterol domain structure occurs in cataracts. In this study, fiber cell plasma membranes were isolated from both normal (control) and cataractous lenses and assayed for cholesterol and phospholipid. Control and cataractous whole lens membranes had C/P mole ratios of 3.1 and 1.7, respectively. Small angle x-ray diffraction approaches were used to directly examine the structural organization of the cataractous lens plasma membrane versus control. Both normal and cataractous oriented membranes yielded meridional diffraction peaks corresponding to a unit cell periodicity of 34.0 A, consistent with the presence of immiscible cholesterol domains. However, comparison of diffraction patterns indicated that cataractous lens membranes contained more pronounced and better defined cholesterol domains than controls, over a broad range of temperature (5-40 degrees C) and relative humidity (52-92%) levels. In addition, diffraction analyses of the sterol-poor regions of cataractous membranes indicated increased membrane rigidity as compared with control membranes. Modification of the membrane lipid environment, such as by oxidative insult, is believed to be one potential mechanism for the formation of highly resolved cholesterol domains despite significantly reduced cholesterol content. The results of this x-ray diffraction study provide evidence for fundamental changes in the lens fiber cell plasma membrane structure in cataracts, including the presence of more prominent and highly ordered, immiscible cholesterol domains.

Endogenous sodium pump inhibition: current status and therapeutic opportunities

One might ask, given the number of false trails that have been pursued, why we, and so many others, have continued to pursue the elusive digitalis-like factor? The answer can be found in the many review articles cited above [4-13]. In animal models of volume-dependent hypertension, evidence favoring sodium pump inhibition as at least a contributing factor, is essentially overwhelming. These observations are supported by multiple lines of less direct evidence in humans which are also compatible with a contribution of a circulating sodium pump inhibitor. Indeed, if multiple premature claims announcing the isolation of the digitalis-like factor had not appeared, this would be one of a large number of interesting scientific areas in which identification of a responsible vector was expected momentarily. The disenchantment so often expressed, we believe, reflects a response to those premature claims. We echo a recent review on the digoxin-like sodium pump inhibitor story from one of the productive groups in this area. "Now that there is little doubt that endogenous digoxin-like inhibitors of sodium transport exist..., the link between these substances, salt intake and vascular tone must be pursued with increasing vigor" [12]. That pursuit, of course, will be easier if the criteria concerning the responsible mediator are employed systematically. Because the current situation resembles so strikingly the situation late in the nineteenth century--when efforts focused on the attempt to identify a specific microorganism as the agent responsible for specific disease--we employed Koch's Postulates as the organizing principle. The challenge faced by Robert Koch over a century ago is identical to the challenge that those of us who are interested in digitalis-like factors face today. Passionate advocacy and equally impassioned denial can be seen as a stage in the scientific process when the problem is important and has proven to be more intractable than anticipated. Substantial, but still circumstantial evidence supports strongly a role for a circulating digitalis-like factor not only in normal sodium homeostasis and in the pathogenesis of salt-sensitive hypertension, but also in the pathogenesis of a wide array of processes that have an uncertain etiology. Although supported by many lines of evidence, this intriguing concept remains controversial, in large part because the responsible factor has proven to be very elusive. Informed opinion today ranges from arguments that the agent does not exist to contrary arguments that the agent has been identified. A very large number of candidates from a wide range of chemical classes have been proposed. Indeed, the large number of candidates, none supported by absolutely definitive evidence, has contributed to the controversy. In this essay, we have attempted to define and illustrate the information that will be required before a candidate becomes widely accepted.

A hypothetical mechanism for toxic cataract due to oxidative damage to the lens epithelial membrane

Lenticular opacities can be induced by numerous external agents that coincide with those that catalyze oxidative damage to lipids. One of the consequences of lipid peroxidation is that the affected membrane is rendered more permeable to protons. A proton leak in the tight epithelium of lens would uncouple the Na+/K(+)-ATPases that regulate the water and ionic content of the bounded tissue. Once regulatory control of the osmotic pressure is lost, the phase state of the cell's soluble proteins would change, leading to refractive changes or, in extreme cases, precipitation. The same does not occur in cornea because the stroma is an extracellular polymer blend rather than solution of soluble polymers. Polymeric phase transitions in the cornea require that divalent cations pass the epithelial membrane, which can occur only through the action of ionophores.

The dual effect of oxidation on lipid bilayer structure

Sphingomyelin membranes were prepared with different levels of oxidative damage caused by tert-butyl hydroperoxide (TBH). Temperature-induced changes in membrane hydrocarbon chain packing (phase transitions) were monitored using infrared spectroscopy. Lipid phase transition characteristics were evaluated from thermodynamic parameters fitted to the experimental transition curve data. At temperatures below the lipid phase transition Tc, hydrocarbon chains pack in an ordered state whereas above the Tc the hydrocarbon chains pack in a disordered state. Compared to the non-oxidized control, the packing of the hydrocarbon chains of mildly oxidized sphingomyelin (less than 10 nmol TBH/mg lipid) was no different at all temperatures below the Tc, and was more ordered above the Tc. The hydrocarbon chains of strongly oxidized sphingomyelin (greater than 10 nmol TBH/mg lipid) were more disordered at temperatures above and below the Tc compared to the control samples. These results suggest that lipid oxidation has a dual effect on lipid order. A more ordered or disordered state may result depending on the degree of oxidation and the state of lipid order prior to oxidation. These results could be important for explaining the structural changes in oxidized membranes high in sphingomyelin such as those found in the ocular lens and liver plasma membranes.

Na,K-ATPase of cultured bovine lens epithelial cells: H2O2 effects

Na,K-ATPase function was studied in cultured bovine lens epithelial cells under confluent and non-confluent conditions. The affinity of the Na,K-ATPase for the cardiac glycoside, ouabain, differs between the confluent and non-confluent cultures. The confluent cells have a higher affinity for ouabain than do the non-confluent cells. The ouabain affinity of the confluent cells is similar to that for the Na,K-ATPase isolated from the bovine axolemma and the bovine lens cortex. The ouabain affinity of the non-confluent cells is similar to that for the Na,K-ATPase of the renal medulla and bovine lens epithelium. Similar results are not found with confluent and non-confluent MDCK cells. H2O2 treatment of confluent and non-confluent lens epithelial cell cultures has differing effects on the Na,K-ATPase function. In the confluent cell preparations, H2O2 affects K(+)-dependent dephosphorylation of the intermediate phosphoenzyme. In the non-confluent preparations. H2O2 appears to inhibit K(+)-occlusion.

The effect of near-UV light on Na-K-ATPase of the rat lens

The influence of in vitro near-UV radiation exposure on the physical state of the rat lens and on its membrane-bound Na-K-ATPase activity was investigated. Lens swelling was correlated to the appearance of opacities and the inactivation of the enzyme. The results show a significant decrease in the Na-K-ATPase activity which may be an early change leading to osmotic type cataracts. The dose-effect curves obtained for cortical and epithelial enzymes were different. Since the data do not follow a monoexponential function, the existence of two forms of Na-K-ATPase in the lens is discussed.

Pulse techniques for the suppression of individual components in multiexponential relaxation curves

NMR pulse sequences have been investigated which selectively suppress undesired signal components from multiple-exponential spin-spin and spin-lattice relaxation curves. Suppression is achieved by exploiting differences in the relaxation times of sample components. Special attention is devoted to discussion of practical details necessary for implementation of these methods, as well as their relative advantages and disadvantages. The techniques are illustrated using multiple-compartment aqueous phantoms and are then employed in measurements of proton T1 and T2 values for freshly isolated guinea pig lenses immersed in aqueous buffer solutions to prevent dehydration and maintain viability. These methods obviate many of the difficulties inherent in obtaining accurate relaxation times for minor components of multiple-exponential curves and should be useful in many applications of NMR to the study of intact tissue, both in vivo and in vitro.

ATP hydrolysis kinetics of Na,K-ATPase in cataract

The steady-state kinetics of hydrolysis of Mg2+ ATP by the epithelial Na,K-ATPase of individual human lenses were determined. Among the cataract lens population, four distinct kinetic types were observed: negative kinetic co-operativity. Michaelis-Menten kinetics, positive kinetic co-operativity, and substrate inhibition kinetics. Negative kinetic co-operativity and Michaelis-Menten kinetics were also observed in a group of presumably clear lenses from non-diabetic individuals ages 16-42 years. Substrate inhibition kinetics were found to be prevalent in individuals with mature onset diabetes. Substrate inhibition kinetics were also observed for Na,K-ATPase isolated from lenses which had been incubated in high glucose. It would appear that this modification leads to an inhibition of Na,K-ATPase-dependent K+ influx into these cultured lenses.

Membrane permeability characteristics of perfused human senile cataractous lenses

Human cataractous lenses were removed by the cryoprobe technique and were maintained for up to 24 hr in a solution of similar ionic composition to human aqueous humour. The bimodal distribution of internal sodium concentrations was similar to that previously reported for unincubated human lenses. Lenses with lower total and free sodium contents had relatively higher membrane potentials and they lost 86Rb at a slower rate than lenses with high internal sodium. The 86Rb efflux in these lenses was stimulated four-fold by removing external calcium. The efflux was reduced by increasing external calcium, but was increased during a small (60 mosmol) hyperosmotic shock. A similar hyperosmotic shock also surprisingly increased 86Rb efflux. Lenses with increasing internal sodium (and calcium) levels showed an increasing rate of loss of 86Rb and the stimulation by calcium removal was progressively diminished. The efflux from lenses with disturbed ion levels was also relatively insensitive to changes in external osmolarity and to increasing external potassium. Lenses with raised sodium concentrations also had an increased inulin space. Frog, rat and rabbit lenses were also exposed to the same range of stimuli and only frog lenses responded to the low calcium solution with more than a four-fold increase in efflux rate. Although only a two- to four-fold increase in efflux rate was obtained from rabbit lenses exposed to Ca-free conditions, this was the only type of animal lens so far tested that, like the human lens, responded to both hyperosmotic and isosmotic shocks with an increase in efflux rate. All three species of mammalian lenses responded with an increase in efflux rate when exposed to a hyperosmotic test solution while in the frog, the efflux rate from the lens decreased. The glucose efflux from human cataractous lenses was inhibited by cytochalasin B in a similar manner to the efflux from rat and frog lenses. It was concluded, therefore, that the cryoprobed human lens can be kept for a limited period in a relative simple artificial aqueous humour solution. The potassium permeability characteristics of low sodium cataracts remained relatively intact and showed a unique response (relative to lenses from other animals) when exposed to various stimuli that are known to be potentially cataractogenic.

Morphological and cell volume changes in the rat lens during the formation of radiation cataracts

Earlier studies in our laboratory showed that 24 hr after X-irradiation, epithelial cells of early postnatal rat lenses increased in volume. Three days after X-irradiation, the underlying lens fibers increased in volume. This finding suggested a correlation between damage to epithelial cell volume regulation and subsequent fiber cell swelling. To test this hypothesis 4-week-old rat lenses were three-dimensionally reconstructed to determine average cell volumes of specific lens regions and wet weights of whole lenses were measured during radiation cataract formation. In addition, the differentiation of epithelial cells into lens fibers was monitored by autoradiography. Four-week-old Sprague-Dawley rats were injected with [3H]-thymidine and, 24 hr later, their eyes were irradiated with either 400 or 1200 rad. Lenses were examined with a slit lamp and cataracts were graded on a scale of 1+ to 4+. Animals were killed 24 hr and 3, 5, 15 and 30 weeks after exposure. Lenses were serially sectioned at 0.75 micron and epithelial, equatorial and cortical fiber cell volumes were determined. Rats exposed to 400 or 1200 rad developed 0.5-1.5+ or 2.5-3.0+ cataracts, respectively, 10-16 weeks after X-irradiation. Epithelial and equatorial cells of both groups did not significantly increase in volume during this period. Three weeks after irradiation with 1200 rad cortical fibers were disorganized and had increased volume. By 5 and 15 weeks, cortical fibers had more normal cell volumes, although their morphology remained grossly altered. Cortical fiber volume of lenses irradiated with 400 rad were not significantly different from control lenses throughout the experimental period. By 15 weeks lenses irradiated with 400 rad showed subtle changes in morphology. Wet weight determinations indicated that the localized increase in cortical fiber volume did not result in an increase in the wet weight of the entire lens. Autoradiography showed that affected fibers had been epithelial cells at the time of X-irradiation. These results provide additional evidence that disturbances in fiber differentiation are involved with cataract formation, but do not support the initial hypothesis that a disturbance in epithelial cell volume regulation leads to fiber cell swelling. Earlier results suggesting defects in lens epithelial volume regulation in radiation cataract formation may have been complicated by ocular inflammation.

86-Rb efflux in normal and cataractous human lenses

86Rb efflux has been studied in normal lenses and in human senile cataracts. The rate constant (Ki) of the efflux gradually increases in cataractous lenses with progression of lens damage. Efflux experiments run in the presence of BaC12 suggest that a progressive activation of BaC12 inhibitable efflux routes occurs in cataractous lenses. In the final stages of opacification the ineffectiveness of BaC12 enriched or Ca++ free media on the efflux suggests that a direct disruption of the lens membranes has occurred.

Effect of glutathione deprivation on lens metabolism

1-Chloro-2,4-dinitrobenzene (CDNB) was used to conjugate glutathione (GSH) through the catalysis of lens glutathione S-transferase without the untoward oxidative damage to the lens mediated by GSH oxidants. A 2 hr treatment of the rat lens with 1 mM CDNB resulted in a nearly total depletion of lens GSH with neither formation of GSSG nor glutathione-protein mixed disulfides. Rubidium uptake was found to decrease linearly with the loss of GSH; nevertheless, ionic imbalance did not commence until more than 30% cation pump activity was lost. Glycolytic rate dropped following CDNB treatment, due probably to a decline in demand for ATP by the deactivated cation pump. 31P-NMR studies confirmed the irreversible loss of ATP. CDNB depletion of GSH resulted in a two-fold increase in 14CO2 production from [14C]-1-glucose. Whereas oxidative stress resulted in a six-fold increase in glucose utilization through the hexose monophosphate shunt (HMPS), CDNB-treated lenses showed no such stimulation. This indicated that the residual GSH following CDNB treatment was insufficient for the activation of the glutathione peroxidase-reductase-HMPS mechanism and raised the possibility that the increased glucose utilization might be due to mechanisms other than the HMPS. These results indicate an intimate correlation between the GSH content and major metabolic functions in the lens.

Amino-acid transport and protein synthesis in the rabbit lens: absence of cryoprobe effect

Recently developed organ-culture techniques have been used to investigate the effects of cryoprobe treatment on rabbit lenses. Uptake of 14C-tyrosine into cryoprobe treated and control lenses was followed for 96 h. Lens proteins were separated by gel filtration and incorporation of label measured in the individual crystallins. The cryoprobe treatment had no measurable effect on lens water, Na+, K+ or Ca++ content, tyrosine transport or the incorporation of tyrosine into the crystallins, during the period of the experiment.

The Emory mouse cataract: accumulation of free leucine and incorporation into protein by cataractous and clear lenses

Leucine metabolism has been studied in the Emory mouse lens to determine if there is a correlation between the rates of leucine accumulation and incorporation and the stage of cataractogenesis. Developing cataracts were graded in vitro according to severity and then incubated in tracer leucine. There was no significant difference between grades with respect to leucine incorporation or accumulation of free leucine, although there was a gradual decrease in CL/CM as the cataracts increased in severity. The results showed a high degree of variability, especially for incorporation, leading to the conclusion that the morphological appearance of these cataracts is not predictive of their metabolic behavior as revealed by leucine accumulation or incorporation.

Molecular mechanism of cataractogenesis: III. Toxic metabolites of oxygen as initiators of lipid peroxidation and cataract

A free radical mechanism of cataractogenesis involving enzymatic and nonenzymatic reactions, is proposed. Supporting experimental evidence is briefly reviewed. H2O2, which is one of the toxic metabolites of oxygen, was significantly increased 2-3 fold in ocular humors in several experimental cataracts and in human senile cataract. Various cataractogenic agents were also found to increase H2O2 in ocular humors in vivo prior to cataract formation. Enzymatic defenses against O2-. and H2O2 provided by superoxide dismutase, catalase and glutathione peroxidase were impaired in cataracts. In some cataracts, catalase and superoxide dismutase were affected earlier. Malondialdehyde (MDA), a major breakdown product of lipid peroxides was significantly increased by 2-4-fold in human senile cataract, in cataracts induced in rabbit and rat, and in hereditary cataracts in mice. All the reactive species of O2 (O2-., H2O2, OH. and 1 delta gO2) may participate in initiating lipid peroxidation of lens in vitro. Various scavengers of these species were capable of preventing lenticular lipid peroxidation, amongst which OH. scavengers were found to be the most effective. Biological antioxidant, vitamin E afforded 44% prevention of lipid peroxidation in lens. The important observation was that vitamin E was therapeutically effective in about 50% of animals in arresting cataract induced in rabbit by 3-aminotriazole. In these rabbits, H2O2 and ascorbic acid of ocular humors and MDA of lens were close to normal. It is our working hypothesis that the carbonyl groups of MDA and amino groups of amino acids, proteins, nucleic acids and their bases, and phospholipids could interact in a cross-linking reaction producing high molecular weight aggregates by Schiff-base conjugate formation in addition to disulfide cross-linking of proteins, and finally resulting in cataract.

Active and passive rubidium influx in normal human lenses and in senile cataracts

The aim of this study was to investigate the mechanisms responsible for the osmotic imbalance which occurs in a majority of human senile cataracts. Active and passive influx of rubidium has been determined in vitro in human lenses both normal and cataractous. It is concluded that the active transport of cations is on the average normal in senile cataractous lenses. It is possible that the activity of the cation pump is defective in a few cataractous lenses but no direct evidence of this can be given. The results indicate that cryo-extracted lenses may be utilized in this type of study provided that cryo-treatment during surgery is kept at the lowest possible level.

Sodium/potassium ATPase in normal and cataractous human lenses

Na/K ATPase activity has been determined in normal and cataractous human lenses using labeled ATP as a substrate. The enzyme is distributed throughout the lens with approximately 1/3 of the total activity in the capsule-epithelium and 1/2 of the activity in the cortex. Furthermore, the activity of Na/K ATPase decreases with increasing age and this decrease occurs primarily in the inner nuclear region. In severe cataractous lenses, a marked decrease in the Na/K ATPase activity has been demonstrated in all parts of the lens, in contrast to immature cataracts, where the decrease in the enzyme activity occurs primarily in the cortical and nuclear regions.