The C-13 and H-2 isotope Substitution effects have been examined for the "Raman spectroscopic signature of life," a mitochondrial Raman band at 1602 cm(-1) that sharply reflects the metabolic activity of living yeast cells. The band shifts to 1542 cm(-1) with C-13 substitution and to 1599 cm(-1) with H-2 substitution. Normal mode analysis based oil a DFT calculation suggests that it originates from a C=C double bond having no hydrogen atoms attached to the carbon atoms. The in-phase C=C stretch mode of ubisemiquinone radicals (CoQ and/or CoQH*) emerges as a strong candidate for the origin of the 1602 cm(-1) band.