Establishing a Low Redox Potential in Giant Yeast Colonies: Effects of Media and Rotation

Holly H. Birdsall, Patricia L. Allen, Jeffrey S. Hammond, Timothy G. Hammond

Abstract


Giant yeast colonies develop a low redox potential, which mimics the electrophilic milieu of both the mitochondrial drug metabolizing compartment, and the hypoxic core of many tumors. The major metabolic mediators of this low redox potential include ATP, glutathione, NAD+/NADH and NADP+/NADPH. Ammonia signaling is the critical mechanism that induces stratification of the giant yeast colonies to allow a low redox potential. Comparison of two powerful investigative models for drug pathways using Saccharomyces cerevisiae have been compounded by the use of different growth media and stimuli to the system. Chemogenetic profiling, which uses a pool of yeast deletion mutants to determine survival changes, is heavily slanted to the use of rich media. Giant yeast colonies studies are heavily slanted to the use of poor media. The current study answers three questions. First, what are the differences in redox potential and its major metabolic mediators in giant yeast colonies over time? Metabolic status is assayed as glutathione content, redox potential, ATP content, and ratios of NAD to NADH and NADP to NADPH. Second, how does rich versus poor media affect physiological responses of giant yeast colonies? Third, how does clinorotation affect redox potential and its metabolic mediators? Clinorotation is of interest because it randomizes the gravitational vector. That in turn should disrupt the convection-medicated ammonia signaling known to drive giant yeast colony stratification and differentiation. This study demonstrates that the low redox potential status in tumors and drug metabolizing areas of mitochondria can be reproduced in giant yeast colonies grown on both poor and rich media. The study defines the time course and effects of middle molecule mediators from glutathione and ATP to the NAD family of mediators. Rotation proved to be a potent way to selectively modulate ammonia signaling, redox potential, and metabolic mediators.

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