Fig 7. A model for the function of tetrahydrobiopterin (BH4) in the regulation of appetite.

Punch and Purple are required in the fat body to generate 6-pyruvoyltetrahydropterin (PTP) from guanosine triphosphate (GTP). In our model, PTP is then transferred to the brain where it is converted to BH4 in the neuropeptide F (NPF) neurons. Through an as yet unknown mechanism, BH4 seems to reduce NPF release, in turn, reducing feeding. Solid arrows indicate single-step enzymatic conversions and dotted lines indicate what are likely multi-step processes.