A Comparison of Light-Activated Enzymes From Vertebrate and Invertebrate Photoreceptors
Calhoun, Roger Dale
This item is only available for download by members of the University of Illinois community. Students, faculty, and staff at the U of I may log in with your NetID and password to view the item. If you are trying to access an Illinois-restricted dissertation or thesis, you can request a copy through your library's Inter-Library Loan office or purchase a copy directly from ProQuest.
Permalink
https://hdl.handle.net/2142/70705
Description
Title
A Comparison of Light-Activated Enzymes From Vertebrate and Invertebrate Photoreceptors
Author(s)
Calhoun, Roger Dale
Issue Date
1983
Department of Study
Physiology and Biophysics
Discipline
Biophysics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biophysics, General
Abstract
The three themes of the thesis are: (1) the search for light-activated enzymes in octopus photoreceptors analogous to those found in vertebrates' photoreceptors, (2) the question of which photointermediate(s) of photoexcited rhodopsin activate bovine photoreceptor GTPase, and (3) the calcium and pH dependence of phosphodiesterase in neurons of the mollusc Pleurobranchaea.
GTPase, ATPase, and cGMP and cAMP phosphodiesterase were assayed in octopus and squid photoreceptors. GTP cyclase and ATP cyclase were also assayed in octopus photoreceptors. Of these enzyme activities, only GTPase was found to be light-activatable. The octopus GTPase differs from the bovine light activated GTPase in that (1) it is more tightly bound to the membrane, (2) it is unstable at temperatures > 20(DEGREES)C, (3) greater light activation occurs at somewhat lower magnesium and higher potassium concentrations, and (4) light activation decays much more rapidly in the dark. Similarities between the bovine and octopus GTPases include (1) a Km of < 1 (mu)M GTP, (2) light activation is independent of temperature below 20(DEGREES)C, (3) magnesium is required for activity, and (4) greatest light activation at about pH 8.
Activation of bovine GTPase by photoexcited rhodopsin was investigated. Opsin does not activate GTPase. Conditions which favor increased amounts of metarhodopsin I and reduce the amount of metarhodopsin II decrease activation of GTPase. Assays in the presence of hydroxlyamine to prevent the formation of metarhodopsin III show no effect on activity in light, where formation of metarhodopsin II is continuous. The ability of photoexcited rhodopsin to activate GTPase decays biphasically in the dark, with half-times corresponding to the decays of metarhodopsin II and metarhodopsin III. The equilibrium between metarhodopsin II and metarhodopsin III does not allow an interpretation of the decay as evidence of activation by metarhodopsin III. Thus, metarhodopsin II can activate GTPase, metarhodopsin I probably does not, and metarhodopsin III is not necessary for activation, but may be able to do so.
Sensory input to the command neurons in Pluerobranchaea which drive feeding behavior apparently raises cAMP levels in the command neurons, causing depolarizations and subsequent bursts of action potentials. cAMP levels may then be reduced by a Ca('+2) - calmodulin stimulated phosphodiesterase. The Ca('+2) activation of the phosphodiesterase is very pH sensitive. pH is also changing during the bursting activity.
Use this login method if you
don't
have an
@illinois.edu
email address.
(Oops, I do have one)
IDEALS migrated to a new platform on June 23, 2022. If you created
your account prior to this date, you will have to reset your password
using the forgot-password link below.
