Concentration
Explanation - Biochemical reactions in a cell are catalyzed by enzymes. These enzymes increases the rate of reaction but themselves remain chemically unchanged. Enzymes or biological catalysts are very particular about the PH and salt concentration of the environment in which they are working.
They do not perform their function efficiently if the optimum PH is not maintained. Salts can be potassium based, sodium based, calcium based etc. Therefore, the activity of the enzyme in a biochemical reaction is highly regulated by the concentration of salt present.