Effects of temperature on the spectral properties of visual pigments
were measured in the physiological range (5–28°C) in
photoreceptor cells of bullfrog (Rana catesbeiana) and crucian
carp (Carassius carassius). Absorbance spectra recorded by
microspectrophotometry (MSP) in single cells and sensitivity spectra
recorded by electroretinography (ERG) across the isolated retina were
combined to yield accurate composite spectra from ca. 400 nm
to 800 nm. The four photoreceptor types selected for study allowed
three comparisons illuminating the properties of pigments using the
dehydroretinal (A2) chromophore: (1) the two members of an A1/A2
pigment pair with the same opsin (porphyropsin vs. rhodopsin
in bullfrog “red” rods); (2) two A2 pigments with similar
spectra (porphyropsin rods of bullfrog and crucian carp); and (3) two
A2 pigments with different spectra (rods vs.
long-wavelength-sensitive (L-) cones of crucian carp). Qualitatively,
the temperature effects on A2 pigments were similar to those described
previously for the A1 pigment of toad “red” rods. Warming
caused an increase in relative sensitivities at very long wavelengths
but additionally a small shift of λmax toward shorter
wavelengths. The former effect was used for estimating the minimum
energy required for photoactivation (Ea) of the
pigment. Bullfrog rod opsin with A2 chromophore had
Ea = 44.2 ± 0.9 kcal/mol, significantly
lower (one-tailed P < 0.05) than the value
Ea = 46.5 ± 0.8 kcal/mol for the same
opsin coupled to A1. The A2 rod pigment of crucian carp had
Ea = 42.3 ± 0.6 kcal/mol, which is
significantly higher (one-tailed P < 0.01) than that of the
L-cones in the same retina (Ea = 38.3 ± 0.4
kcal/mol), whereas the difference compared with the bullfrog A2 rod
pigment is not statistically significant (two-tailed P =
0.13). No strict connection between λmax and
Ea appears to exist among A2 pigments any more than
among A1 pigments. Still, the A1 → A2 chromophore substitution in
bullfrog opsin causes three changes correlated as originally hypothesized
by Barlow (1957): a red-shift of
λmax, a decrease in Ea, and an increase
in thermal noise.