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Editor,—A new protease enzyme in the human lens, dipeptidase, has been purified to homogeneity and characterised recently by Sulochana et al.1This enzyme is specific for dipeptides in its protease function while two other lens proteases, leucine amino peptidase2 and amino peptidase III,3 can act on di-, tri-, and oligopeptides. It is reported that the activity of amino peptidase III did not change with age but the specific activity decreased in human cataract lenses.4 Two viewpoints on the role of such proteases have been discussed by earlier workers—namely, a decline in the proteolytic enzymes with age leading to an accumulation of degraded proteins5 and an excessive proteolysis destroying essential structural proteins.6 A study was carried out to find the activity and specific activity of dipeptidase in cataract and aged lenses. The enzyme activity was measured using Leu-Try as substrate by following the procedure described earlier.1Lenses from human donor eyeballs and the operation theatre of Sankara Nethralaya, Vision Research Foundation were used for the analysis.
Activity and specific activity of human lens dipeptidase (normal and cataract) were compared with different age groups and are given in Tables 1 and 2. Though the activity tends to increase with age in the donor eyeball group, the increase is not statistically significant. There is not much difference in the activity among various age groups studied in cataract lenses. However, the specific activity in non-cataract lenses of donor eye in the age group 31–40 is only 0.159 unit/mg protein but that of cataract lens is almost double, at 0.33. Likewise, there is also a statistically significant increase in the specific activity in the cataract group compared with normal in the age groups 41–50 and 51–60.
The specific activity of an enzyme is a better index for assessment of enzyme function than the activity. This increase in specific activity was not only related to aging but also to cataract. Increased specific activity indicates that the dipeptidase enzyme may be a vital protein. Though many other lens proteins undergo either denaturation or precipitation, this enzyme remains viable and, in this context, it is of interest to note that the specific activities of exopeptidases are increased in cataract lenses.6 The specific activity of dipeptidase for three age groups 31–40, 41–50, and 51–60, has increased for cataractous lenses compared with that of donor eyeball. The differences are statistically significant. This clearly indicates that the specific activity increases with cataractogenesis. It is also interesting to note that even among the donor eyes, the specific activity 0.159, for the age group 31–40, increased progressively in age groups 41–50, 51–60, and 61–70, and is 0.413 for the age group 71–80 suggesting its role in advancing age.