Answer;
Muscle cells in oxygen deprivation convert pyruvate to lactate, and in this step gain NAD+.
Explanation;
When oxygen is insufficient or not present the muscle cells for further oxidation of pyruvate and NADH produced in glycolysis, NAD+ is regenerated from NADH by reduction of pyruvate to lactate.
Pyruvate is converted to lactate by the enzyme lactate dehydrogenase.
