Table 1. Tryptic Sec-containing peptides of SelP isolated from HepG2 media samples.

Sample	Tryptic Peptide	Cys, %	Sec, %
Control	TGSAITUQCK	0	100
	ENLPSLCSUQGLR	0	100
	AEENITESCQUR	18.2	81.8
Selenite	TGSAITUQCK	0	100
	ENLPSLCSUQGLR	0	100
	AEENITESCQUR	0	100
Thiophosphate	TGSAITUQCK	100	0
	ENLPSLCSUQGLR	100	0
	AEENITESCQUR	95.5	4.5

Percent Sec and Cys in SelP peptides was assessed by MS/MS as described in Materials and Methods.