The NADP dependent reduction of PAPS into sulfite involves thioredoxin which probably plays the role of a thiol carrier. [thioredoxin]-disulfide + adenosine 3',5'-bisphosphate + 2 H(+) + sulfite = 3'-phosphoadenylyl sulfate + [thioredoxin]-dithiol Sulfur metabolism; hydrogen sulfide biosynthesis; sulfite from sulfate: step 3/3. Belongs to the PAPS reductase family. CysH subfamily. Truncated N-terminus.