(PROTEIN) WATER MOLECULE AMINO GROUP

Proteins • 31
Figure 2–15
The aggregation of lens proteins proposed to occur in senile cataract formation. ➤ Both nuclear and cortical cataracts aggregate as
at (A). The aggregated crystallins (white blobs) are seen bound to each other by disulfide (or other) bonds. They are also bound to 43K
and 26K proteins at the plasma membrane of the lens fiber cells. In cortical cataracts, the aggregation proceeds to cause a rupture in
the membrane (arrow at B), which destroys lens fiber cells and produces cellular debris as seen at (C). (Adapted from Spector A, 1984)
the lens absorbs these wavelengths. As a result of this, a possible connection
between such ultraviolet light and tryptophan alteration was
suggested by Pirie (1971, 1972) who reported that ultraviolet light was
able to oxidize tryptophan to N’-formylkynurenine in proteins in vitro
(Figure 2–17 A and B). She thought that such altered amino acids in crystallins
may lead to the yellow to brown coloration in nuclear cataracts.
It was also indicated that this process might eventually lead to the
nondisulfide crosslinking that is characteristic of HM4 aggregates found
in nuclear cataracts. Some candidate substances, found in lenses, which
have been proposed to link tryptophan with nondisulfide crosslinking
are anthranilate and β-carboline (Garcia-Castineiras, Dillon, Spector,
1978; Truscott, Faull, Augusteyn, 1977) (see Figure 2–17 C and D).
Subsequently, van Heyningen (1973) demonstrated that hydroxykynurenine
instead of kynurenine was the metabolite that could be linked to the
interference of light passing through the lens acting as a UV filter.
Although there has been considerable confusion about the role of
hydroxykynurenine in subsequent years, Wood and Truscott (1993),