Catalyzes the reversible isomerization between hydroxypyruvate and 2-hydroxy-3-oxopropanoate (also termed tartronate semialdehyde). 3-hydroxypyruvate = 2-hydroxy-3-oxopropanoate Up-regulated by exposure to 7,8-Dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) after which is found in the culture medium of amniotic epithelial cells. Belongs to the hyi family. Truncated N-terminus.