Abstract
1. 1. The stability of the native conformation of the heme crevice of pyridoxal phosphate (PLP)-ferricytochromes c as assayed by the p K a for 695 nm absorption band varies considerably. The p K a , values are 8.76 for cytoehrome c modified by PLP at lysine 79 [PLP(Lys 79)-cyt. c], 9.23 for cytochrome c modified by PLP at lysine 86 [PLP(Lys 86)-cyt. c], 9.34 for doubly PLP substituted cytochrome c at lysines 79 and 86 [(PLP) 2-cyt. c], 9.50 for triply substituted cytochrome c [(PLP) 3-cyt. c] and 9.06 for native cytoehrome c, which indicates less stable heme crevice of PLP-cytochrome c. 2. 2. The singly PLP-modified cytochrome c indicate decreased activities with mitochondrial cytoehrome c oxidase in the following order: PLP(Lys 86)-cyt. c < PLP(Lys 79)-cyt. c > native cytochrome c. The high affinity K m for PLP(Lys 86)-cyt. c, PLP(Lys 79)-cyt. c and native cytochrome c are 0.28 μM, 0.16 μM and 0.02μ M respectively. 3. 3. PLP-cytochromes c show decreased binding affinities to fluorescence probes 12-(9-antroyl)-stearic acid and pyrene-labelled mitoplasts. The quenching of singly PLP-modified cytochrome c depends significantly on the ionic strength.
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