PLANT PHYSIOLOGY , Vol 101, Issue 3 947-953, Copyright © 1993 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
Ultraviolet-Induced Photodegradation of Cucumber (Cucumis sativus L.) Microsomal and Soluble Protein Tryptophanyl Residues in Vitro
C. R. Caldwell
Climate Stress Laboratory, United States Department of Agriculture, Agricultural Research Service, Beltsville, Maryland 20705-2350
The in vitro effects of ultraviolet B (280-320 nm) radiation on microsomal
membrane proteins and partially purified ribulose bisphosphate carboxylase
(Rubisco) from cucumber (Cucumis sativus L.) was investigated by measuring
the direct photolytic reduction of tryptophan fluorescence and the
formation of fluorescent photooxidation products. Exposure of microsomes
and Rubisco to monochromatic 300-nm radiation resulted in the loss of
intrinsic tryptophan fluorescence and the production of blue-emitting
fluorophores. The major product of tryptophan photolysis was tentatively
identified as N-formylkynurenine (N-FK). Even though the rates of
tryptophan photodegradation and N-FK formation were similar, the amount of
blue fluorescence produced was significantly higher in the microsomes
relative to Rubisco. Studies with various free radical scavengers and other
modifiers indicated that tryptophan photodegradation requires oxygen and
that the subsequent formation of N-FK may involve reactive oxygen species.
The optimum wavelengths for loss of typtophan fluorescence were 290 nm for
the microsomes and 280 nm for Rubisco. The temperature dependence of
tryptophan fluorescence and rate of tryptophan photodegradation indicated
an alteration in the cucumber microsomal membranes at about 24[deg]C, which
influenced protein structure and tryptophan photosensitivity.
