There isn't really a single perfect synonym for "metabolism," as it encompasses a wide range of processes. However, depending on the context, you could use:

General synonyms:

* Biochemical activity

* Chemical processes

* Physiological activity

* Cellular activity

* Life processes

More specific synonyms:

* Anabolism (for building up processes)

* Catabolism (for breaking down processes)

* Metabolisation (focuses on the act of converting substances)

* Biotransformation (for chemical changes in the body)

The best word to use will depend on the specific context and what aspect of metabolism you want to emphasize.