Ruth van Heyningen

Formation of Polyols by the Lens of the Rat with |[lsquo]|Sugar|[rsquo]| Cataract

CATARACT (opacity) of the lens inevitably develops in young rats fed xylose or galactose1, or in young diabetic rats if the blood glucose is maintained at a level above 2.5 mgm./ml. blood2. These three forms of cataract which resemble each other closely in clinical and histological appearance, are known as ‘sugar’ cataracts because in all cases there is a raised level of monosaccharides in the blood. The early metabolic changes within the lens, which lead to gradual loss of transparency, are unknown. It was decided to examine the carbohydrates and their derivatives in normal and cataractous lenses to see if changes occurred in ‘sugar’ cataract, and if so whether there was any similarity in this respect between the three forms of cataract.

Fluorescent Glucoside in the Human Lens

A PROTEIN-FREE extract of the lens of man and other primates contains a fluorescent compound which is absent from the lens of the usual domestic and laboratory animal. This compound, detected on paper chromatograms by its reaction with ninhydrin, was believed to be a peptide1. The protein free extract of the lens of man and certain other primates has been found to contain one or more fluorescent yellow pigments2, and I have now isolated from the human lens the O-β-D-glucoside of L-3-hydroxykynurenine (Fig. 1), which is yellow and highly fluorescent. This glucoside has also been found in the silkworm, Bombyx mori3.

The metabolism of naphthalene and its toxic effect on the eye

1. Naphthalene (1g./kg.) was fed daily by stomach tube to rabbits. 2. In more than half of the rabbits opacities in the lens and degeneration of the retina were visible in vivo. 3. Dissection of eye tissues revealed some or all of the following changes: a browning of the lens and eye humours, blue fluorescence of the eye humours and crystals in the retina and vitreous body. 4. The ascorbic acid concentration of the eye humours was decreased. 5. Some metabolites of naphthalene [1,2-dihydro-1,2-dihydroxynaphthalene, 2-hydroxy-1-naphthyl sulphate and (1,2-dihydro-2-hydroxy-1-naphthyl glucosid)uronic acid] are converted enzymically by the tissues of the eye into 1,2-dihydroxynaphthalene. 6. Changes in the eye are consistent with 1,2-dihydroxynaphthalenes being the primary toxic agent. The properties and reactions of this substance are described. 7. 1,2-Dihydroxynaphthalene is readily autoxidizable in neutral solution to form the yellow 1,2-naphthaquinone and hydrogen peroxide. This oxidation is reversed by ascorbate. 8. Ascorbate is oxidized catalytically by 1,2-naphthaquinone. This may account for the disappearance of ascorbate from the aqueous and vitreous humours of the eye after naphthalene feeding. It may also account for the appearance of crystals of calcium oxalate in the eye. 9. The brown colour of the lens of the naphthalene-fed rabbit is due to presence of naphthaquinone-protein compounds.

THE EFFECT OF DIABETES ON THE CONTENT OF SORBITOL, GLUCOSE, FRUCTOSE AND INOSITOL IN THE HUMAN LENS.

The lens of a diabetic contains more sorbitol, glucose and fructose than that of a non-diabetic. This finding, which applies both to senile cataractous lens and to non-cataractous lens obtained post mortem, is considered in relation to theories of the cause of formation of cataract in diabetes. There is an inverse relationship between the inositol and the glucose content of the senile cataractous lens.

The human lens. I. A comparison of cataracts extracted in Oxford (England) and Shikarpur (W. Pakistan).

Abstract Cataracts removed at operation in Oxford (England) and Shikarpur (West Pakistan) were compared with respect to appearance, wet weight, dry weight and sodium concentration. The Shikarpur cataract was generally the more advanced, having a higher sodium concentration and a lower percentage dry weight. The implications of the widely varying amounts of protein per lens are discussed. Crystalline calcium phosphate was identified in plaques located in Shikarpur cataracts.

The human lens: III. Some observations on the post-mortem lens

Abstract Changes in the dry and wet weights with age have been measured in a series of post-mortem lenses received at autopsy from patients over 50 years old, in Oxford. The dry and wet weights increase with age, while the percentage dry weight is about the same, between 29·8 and 31·3%, (average 30·8%). Because the post-mortem lens may not have been removed until 1–3 days after death it has an abnormally high sodium content and has probably taken up water. After the age of 30, the post-mortem lens does not become drier with age; the lens nucleus has about 35% dry weight, from the age of 31 to 92 years. The decrease in water content from the outer layers through to the nucleus is less steep in the human lens than in the lens of the cow, rabbit or rat.

The sorbitol pathway in the lens

The evidence for the operation of the sorbitol pathway, which converts glucose to fructose by way of sorbitol, and its possible function in the lens, are reviewed. The mechanism whereby the normal course of the pathway is interfered with in sugar cataract is discussed.

Photo-oxidation of lens proteins by sunlight in the presence of fluorescent derivatives of kynurenine, isolated from the human lens.

Abstract Human lens proteins were exposed to sunlight in aqueous solution at pH 8·0–8·25, with and without the addition of lens fluorescent compounds (LFC). LFC are found in the human lens; the main constituents are derivatives of kynurenine. In the presence of LFC the proteins became yellow-brown, whereas in their absence there was only a faint yellowing. The sunlight caused a decrease in the absorption of the proteins at 278 nm, and an increase below 265 nm and above 295 nm; these changes were greater in the presence of LFC. Tyrosine was photo-oxidized only in the presence of LFC; the photo-oxidation of tryptophan was greater in their presence. Methionine, histidine and cysteine were partly destroyed with and without LFC. The proteins irradiated in the presence of LFC were more resistant to complete proteolysis than the controls. Less than half of the amount of free amino acids was liberated and peptides resistant to hydrolysis remained. In the absence of LFC hydrolysis by proteolytic enzymes seemed to be almost complete. The evidence shows that, under certain circumstances, LFC accelerates the photo-oxidation by sunlight of human lens proteins, probably by acting as a photosensitizer.

An investigation into the loss of proteins of low molecular size from the lens in senile cataract.

Some evidence is given that does not contradict the idea that low molecular weight proteins are lost from the senile cataractous lens by leakage.

Assay of fluorescent glucosides in the human lens

Abstract The concentrations of three fluorescent compounds in the human lens ( van Heyningen, 1971 ), F2 (0-β- d -glucoside of 3-hydroxy- l -kynurenine), F1 (0-β- d -glucoside of a compound closely related to 3-hydroxy- l -kynurenine) and F3 ( l -kynurenine) were measured in the human lens of various ages, in senile cataract, and in the separated lens cortex and nucleus. F2 is at a concentration of 0·7 to 4·0 μ m /g lens, F1 at 0·06 to 0·4 μ m /g lens and F3 at 0·01 to 0·1 μ m /g lens. Values in cortex and nucleus are similar and there are no great changes with age. The fluorescent compounds probably generally decrease in concentration in senile cataract.