Oxidase that catalyzes the conversion of cysteine to 3- oxoalanine on target proteins, using molecular oxygen and an unidentified reducing agent (PubMed:12757706, PubMed:15657036, PubMed:15907468, PubMed:16368756, PubMed:21224894, PubMed:25931126). 3- oxoalanine modification, which is also named formylglycine (fGly), occurs in the maturation of arylsulfatases and some alkaline phosphatases that use the hydrated form of 3-oxoalanine as a catalytic nucleophile (PubMed:12757706, PubMed:15657036, PubMed:15907468, PubMed:16368756, PubMed:25931126). Known substrates include GALNS, ARSA, STS and ARSE (PubMed:12757706, PubMed:15657036, PubMed:15907468). {ECO:0000269|PubMed:12757706, ECO:0000269|PubMed:15657036, ECO:0000269|PubMed:15907468, ECO:0000269|PubMed:16368756, ECO:0000269|PubMed:21224894, ECO:0000269|PubMed:25931126}. This is the function of SUMF1 (sulfatase modifying factor 1, Ensembl gene identifier ENSG00000144455).