Hi Nazeesh, for heavy metal exposure, I would rather look at serum albumin. Serum albumin in almost all species is very abundant and has a free sulfhydryl that can bind many heavy metals.

Pepsin is a digestive enzyme that tells you what is in the gut. Serum albumin tells you what is in the circulation.

Hi, there are a lot of papers on this on google scholar.

https://scholar.google.com/scholar?hl=en&as_sdt=1%2C23&as_vis=1&q=enzyme+fish+intestine+inhibition+lead+mercury&btnG=

You will have to narrow the search parameters to the metals you want to measure and the tissues.

Sorry I couldn't be more helpful.

This article might help: Article Toxicity, mechanism and health effects of some heavy metals