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|Title:||A Comparison of Light-Activated Enzymes From Vertebrate and Invertebrate Photoreceptors|
|Author(s):||Calhoun, Roger Dale|
|Department / Program:||Physiology and Biophysics|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|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.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1983.
|Date Available in IDEALS:||2014-12-16|
This item appears in the following Collection(s)
Dissertations - Biophysics and Computational Biology
Graduate Dissertations and Theses at Illinois
Graduate Theses and Dissertations at Illinois