Spectroscopic properties of chicken iodopsin were investigated in correlation with the concentration of chloride in digitonin extracts. When chloride in the extract was depleted by extensive dialysis, chloride-depleted iodopsin (absorption maximum, 512 nm) was formed. It was converted to chloride-bound iodopsin (absorption maximum, 562 nm) by the addition of chloride in the extract. There existed an equilibrium between two forms of iodopsin with a dissociation constant of 0.8 mM chloride. The chromophore-transfer reaction from iodopsin to scotopsin or B-photopsin, the protein moiety of chicken rhodopsin or chicken blue-sensitive cone pigment, respectively, in digitonin extract was also investigated in correlation with the concentrations of chloride, other monovalent and divalent anions, and detergent. The chromophore of chloride-depleted iodopsin was easily transferred to scotopsin in the extract, resulting in formation of rhodopsin. On the other hand, chloride-bound iodopsin was fairly stable even in the presence of scotopsin, indicating that the reaction is inhibited by binding of chloride to iodopsin. The chromophore-transfer reaction to B-photopsin was also observed from chloride-depleted iodopsin but not from chloride-bound iodopsin. The reaction was observable in the 10% digitonin extract as well as in the 2% digitonin extract. The reaction was also observed when 25 mM Na2SO4 was present in the mixture instead of NaCl, but was not when 67 mM NaNO3 was present. All these facts suggest that the chloride binding site of iodopsin does not accept a divalent anion such as SO4(2+), but does accept a monovalent anion such as Cl- or NO3-, which causes inhibition of the chromophore transfer.
To assess the clinical value of combined SPECT imaging with I-123-15-(p-iodophenyl)-3-methyl pentadecanoic acid (BMIPP) and thallium-201 (Tl), the findings were compared with those obtained in a stress Tl study and positron emission tomography (PET) with fluorine-18-fluorodeoxyglucose (FDG) in 22 patients with myocardial infarction. In 20 patients who underwent a stress Tl study, among 75 hypoperfused segments, 27 segments exhibited less BMIPP uptake than Tl (discordant segments), and the remaining 48 segments showed a similar decrease in BMIPP uptake (concordant segments). Twenty-two of 27 discordant segments (81%) exhibited redistribution on stress Tl study. On the other hand, only one of the 48 concordant segments had redistribution (p < 0.001). In 10 patients who underwent a FDG PET study, among 33 hypoperfused segments, seven segments were discordant segments, and the remaining 25 segments were concordant segments. Seven of the eight discordant segments (88%) demonstrated an increase in FDG uptake. In contrast, only five of 25 concordant segments (20%) showed increase in FDG uptake (p < 0.01). Thus, the segments showing discordant BMIPP uptake are considered to be ischemic but viable myocardium. We conclude that combined imaging with BMIPP and Tl is a useful mean for evaluating tissue viability in patients with coronary artery disease, but it may underestimate the extent of tissue viability, compared with FDG PET imaging.
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