- Biologically, any rhythmic center that establishes a pace of activity.
- An artificial regulator of rate activity.
- In chemistry, the substance of which the rate of reaction sets the pace for a series of chain reactions; the rate-limiting reaction itself, in a metabolic pathway, the enzyme catalyzing the slowest or rate-limiting reaction in that pathway.
[L. passus, step, pace]
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Examples: glitazone, GI cocktail, etc.