Long term stability of mitochondrial superoxide dismutase activity in NIST diets I–V

Mitochondrial superoxide dismutase (MnSOD) is a ubiquitos, manganese containing enzyme common to most plant and animal cells. In order to determine the stability of an enzyme in frozen diet composites after long term storage, MnSOD activity was measured over a three year period in five different NIST samples. The CuZn SOD activity was below the detectable limits of the method employed. MnSOD activity was found in unhomogenized, well mixed portions of US Diets I and III. Recently, frozen aliquots of the five diet samples, which had been thawed and refrozen in April 1988, were again thawed and analyzed for SOD enzyme activity. The total enzyme activity was reduced from 50 to 60% as compared to that found in April 1988. The effect of extremes in temperature as a result of repetitive freezing and thawing is known to reduce the biological activity of other peptides. The results of this project indicate that NIST frozen diet composite samples will retain enzyme activity for as long as two years, and this may provide an additional organic substance measurable in these diet composite materials.