Mediates tetrahydrobiopterin inhibition of GTP cyclohydrolase 1. This inhibition is reversed by L-phenylalanine. Homopentamer. Forms a complex with GCH1 where a GCH1 homodecamer is sandwiched by two GFRP homopentamers. Interacts with GCH1. Liver and kidney. Somewhat lower level in testis, heart, brain and lung. Belongs to the GFRP family.