Role of proteins and cholesterol in human senile cataractogenesis

Savita Yadav; KP Mistry; UM Rawai

ORIGINAL ARTICLE

Year : 1991 | Volume : 39 | Issue : 1 | Page : 17-19

Role of proteins and cholesterol in human senile cataractogenesis

Savita Yadav, KP Mistry, UM Rawai
Zoology Department, School of Sciences, Gujarat University, Ahmedabad, India

Correspondence Address:
Savita Yadav
Zoology Department, School of Sciences, Gujarat University, Ahmedabad-9
India

Source of Support: None, Conflict of Interest: None

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PMID: 1894337

Abstract

Cataracts are a major cause of blindness in man with far reaching personal, social and economic consequences. The clarity of the lens is dependent upon the maintenance of the integrity of the fiber cell plasma membrane whose important component is cholesterol. In the present study, we have demonstrated that cataract formation influences the cholesterol and protein distribution within the lens.

How to cite this article:
Yadav S, Mistry K P, Rawai U M. Role of proteins and cholesterol in human senile cataractogenesis. Indian J Ophthalmol 1991;39:17-9

How to cite this URL:
Yadav S, Mistry K P, Rawai U M. Role of proteins and cholesterol in human senile cataractogenesis. Indian J Ophthalmol [serial online] 1991 [cited 2022 May 17];39:17-9. Available from: https://www.ijo.in/text.asp?1991/39/1/17/24491

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Introduction

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Cataracts are a major cause of blindness in man with far reaching personal, social and economic consequences. It contributes to the major surgical workload of Ophthalmic surgeons throughout the world. The objective of cataract research is to cure or prevent lens opacities for which a detailed investigation of the human lens is important. The clarity of the lens is dependent upon the maintenance of the integrity of the fiber cell plasma membrane whose important component is cholesterol^[1]. However, lipids play an important role in the development of cataract. Recent studies have shown that lipid peroxidation also play a major role in the development of cataract^[2],[3] Hence, lipids of the lens have an important role to play in understanding the aging processes and differences between normal and cataractous lenses.

In the normal lens, the regularity of the cellular architecture and its transparency is believed to be the result of a spatial order of lens proteins. Lens proteins consist of water soluble proteins (a, j3 and y crystallin) and water insoluble proteins. (albuminoids)^[4]Therefore, in order to understand changes in lens chemistry occurring with the development of cataract it is necessary to study proteins. In the present study, we have demonstrated that cataract formation influences the cholesterol and protein distribution within the lens.

Material and methods

50 human senile nuclear cataractous lenses of age group 50-70 years were analysed. Lenses were kept in normal saline and analysed on the same day after the operation. Nucleus and cortex were analysed separately. An adequate number of normal lenses were not available for analysis therefore the present data is compared with reported normal values.

Biochemical determinations proteins

Protein levels were determined by the procedure of Lowry et al^[5] Absorbancy measurements were made at 600 nm in DU-40 Beckman spectrophotometer.

The procedure of Seyfried et al. was used for extraction of cholesterol in the human lens^[6] Cataractous lenses were weighed and homogenized in chloroform: methanol (1:1) and the contents were filtered through a sintered glass funnel. The residue is again washed with chloroform : methanol Then 0.2 volume of double distilled water was added and mixed well and then kept for phase separation at room temperature. Upper phase was discarded with a Pasteur pipette and the lower phase was washed with 0.4 volume of upper phase reagent which consisted of chloroform : methanol: water (3:38:37v/v/v) and again mixed well. The upper phase formed was again discarded and lower phase was used for the estimation of cholesterol.

Cholesterol

The method of Bowman and Wolf^[7] was used for the estimation of cholesterol. Aliquots of the lipid extract were taken in test tubes and evaporated to dryness at 60^°C in a water bath. 1.5 ml of distilled alcohol was then added to it. 1.5 ml of iron reagent was added while shaking the contents of the tubes on the vertex shaker. The colour developed was read after 30 minutes at 540 nm in DU-40 Beckman spectrophotometer against a reagent blank which contained alcohol instead of sample.

Results

Protein analysis in the human senile nuclear cataractous lenses and their controls are presented in [Table - 1]. Human senile cataractous lenses contain about 70% insoluble proteins and 30% soluble proteins. When the cortex and nucleus were analysed separately it showed an increase in insoluble proteins and decrease in soluble proteins compared to reported normal values. The nuclear portion showed significant decrease (p) in soluble proteins and significant increase in (p) insoluble proteins compared to the cortex. Insoluble proteins are found to be more in the nuclear portion which is more severely affected.

Alterations in cholesterol levels are shown in [Table - 2].

The cortex and nucleus are analysed separately for cholesterol. There is significant increase (p) in cholesterol in the nucleus as compared to the cortex. Cholesterol is found to be more (p) in the nucleus as compared to reported normal values.

Changes in cholesterol with respect to soluble to insoluble protein ratio are given in [Table - 3]. There is no significant increase in the total cholesterol of the whole lens, with increase in insoluble proteins. Initially there is no change in cholesterol with increase in insoluble proteins in the cortex and nucleus. However, at the lowest protein ratio (11-20) there is significant increase (p) in the cholesterol in the nucleus as compared to the cortex. In the nucleus, there is significant increase (p) in cholesterol at the lowest protein (11-20) when compared with cholesterol at highest protein ratio (41-50).

Discussion

The development of nuclear senile cataract is associated with increase in insoluble proteins^[8] Therefore the ratio of soluble to insoluble proteins can be used as in index of lens opacity. Lower the protein ratio, more the insoluble proteins, higher the opacity. Changes in proteins can impair the stability as well as catalytic activity of the enzymes and can lead to formation of aggregates, a characteristic of the aging lens^[9] There is no change in the total proteins in the human senile cataractous lens which show that there is neither leakage nor reduced synthesis. However, the major alteration is conversion of soluble proteins into insoluble proteins. Insoluble proteins are increasingly accumulated in the severely affected area i.e. the nucleus. The generation of water insoluble proteins is a drastic change in chemistry of proteins which can lead to formation of cataract.

Several authors have described the deposition of cholesterol esters in the human sclera during aging^[10] Results show that cholesterol accumulates more in the opaque region i.e. the nucleus when compared to the cortex. This can be due to the large amount of cholesterol present in the membranes of older fibers which accumulate in the nucleus of the lens. Broekhuyse et al^[11]t have shown that it can result in increased viscosity of the lens membrane and may expose the intrinsic membrane proteins to oxidative insult and promote aggregation of proteins by changing their conformation.

As there is a large amount of cholesterol in the nucleus as compared to the cortex therefore, the rigidity of the nuclear membrane is high. This result in alteration of the structural integrity of the nucleus of the lens. It can affect the transport of nutrients across the membrane, which can result in exhaustion of nutrients and accumulation of toxic substances. It has been proposed that lack of nutrition anywhere within the lens could results in the loss of lens transparency^[12] There is a gradual increase in cholesterol but the increase is significant only when insoluble proteins are highest suggesting that cholesterol is affected secondary to protein alterations. Changes in cholesterol might not be due to its abnormal metabolism but can be a result of change in proteins.

Summary

The soluble proteins that are lost appear as insoluble proteins thus resulting in disruption of the spatial order of lens specific proteins which can be accompanied by formation of nuclear cataract. Greater amount of cholesterol can be due to its accumulation in the older fibers present in the nucleus of the lens. With marked increase in insoluble proteins, there is increase in cholesterol thus, causing increased viscosity which in turn, affect the transport of nutrients and result in the loss of transparency of the lens.

References

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2.	Dwivedi, R.S. and Partap VB. 1986. Role of lipid peroxidation and trace metal in cataractogenesis. Ind. J. Ophthalmol. 34. 45.
3.	Bhuyan KC, Bhuyan DK and Podos SM, 1986, Lipid peroxidation in cataract of the human. Life Sciences 38. 1463.
4.	Rink H, Bours J and Hoenders HJ, 1982, Guidelines for the classification of lenses and the characterization of lens proteins. Ophthalmic Res.
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6.	Seyfried TN and Gilbert HG. 1978. Cerebral, cerebellar and brain stem ganglioside in mice susceptible to audiogenic seizures. J. Neurochemistry 31,21.
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8.	Van Haard PM, Kramps JA, Hoenders HJ and Wollensak J, 1978, Development of non-disulfide covalent crosslinks in nuclear cataractogenesis. Interdiscip. Top. Gerontol. 13, 212.
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10.	Brokhuyse RM and Kuhlmann ED. 1972, Lipids in tissues of the eye VI. Sphingomyelins and cholesterol esters in human sclera. Exp. Eye Res. 14. 111.
11.	Broekhuyse RM, Kuhlmann ED and Jap PHK. 1979. Lens membranes IX. Some characteristic of fiber membranes in relation to aging and cataract formation. Ophthalmic Rs. 11. 423.
12.	Goodenough DA. Dick JSBIII and Lyons JE, 1980. Lens metabolic cooperation : A study of mouse lens transport and permeability visualized with freeze-substitution autoradiography and electron microscopy. J. Cell Biol. 86,576.