Figure 3.

Transport modes for a net NAD influx via the peroxisomal NAD carrier. The proposed net import of NAD by the transport protein PXN is strictly accompanied by an export of AMP. (A) NAD import is hypothesized to be supported by an AMP uniporter to provide a peroxisomal AMP counter-substrate pool in the first place. (B) Alternatively, an in silico predicted peroxisomal Nudix hydrolase is postulated to degrade NADH to AMP and reduced nicotinamide mononucleotide (NMNH). Both function as a counter-substrate and deliver a net import of NAD. A NMNH/NAD carrier has not been described and NMNH has not been tested for PXN (N. Linka, personal communication).