Primate-specific plasma protein associated with apolipoprotein L-I (apoL-I)-containing high-density lipoprotein (HDL). Binds hemoglobin with high affinity and may contribute to the clearance of cell-free hemoglobin to allow hepatic recycling of heme iron. Secreted into blood plasma and associated with subtypes of high density lipoproteins (HDL). The uncleaved signal sequence interacts with HDL fluid lipids and mediates incorporation into the HDL particle. Belongs to the peptidase S1 family. Although homologous to serine proteases, it has lost all essential catalytic residues and has no enzymatic activity.