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Volume 272, Number 35, Issue of August 29, 1997 pp. 21745-21750
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

Modulation of Opsin Apoprotein Activity by Retinal
DARK ACTIVITY OF RHODOPSIN FORMED AT LOW TEMPERATURE

(Received for publication, May 15, 1997, and in revised form, June 24, 1997)

From the Department of Biochemistry and Molecular Biology and ^¶ Department of Ophthalmology, SUNY Health Science Center at Syracuse, Syracuse, New York 13210

The bovine opsin apoprotein activates transducin, although at a much reduced level than light-activated rhodopsin (Surya, A., Foster, K., and Knox, B. (1995) J. Biol. Chem. 270, 5024-5031). The ability of retinal to modulate opsin apoprotein activity was investigated using a guanyl nucleotide exchange assay on transducin. 11-cis-Retinal reacted with opsin at 22 °C to (a) reform pigment having maximal absorbance at 500 nm and (b) reduce opsin activity by >80%. Pigment formation also occurred at 0 °C with a t1/2 of 260 min. However, unlike rhodopsin formed at 22 °C (R²²), the rhodopsin formed at 0 °C (R⁰) activated transducin with the same half-saturating concentration as opsin in an exhaustive binding assay. Thus, the formation of a protonated Schiff base associated with 500 nm absorbance does not by itself lead to the inactivation of opsin. The R⁰ conformation was partially inactivated by incubation at 22 °C (t1/2 = 61 ± 9 min), suggesting that it may be an intermediate conformation in the regeneration of rhodopsin.

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