Moreover, we newly identified that downregulated expression of apoB and ghrelin genes were the novel mechanisms for chitosan-affected metabolic responses in vivo. All PCs have in tandem disposition the following structures: Nterminal signal peptide followed by profragment region, conserved subtilisin-like catalytic domain, conserved P-domain and divergent C-terminal tail. PC1/3 initiates the formation of the GW2580 active forms of neuropeptides by cleaving the precursor proteins after pairs of basic residues, the same general and conserved motif recognized by the majority of PCs. PC1/3 is synthesized as a 753-amino acid zymogen, and undergoes autocatalytic intramolecular processing of its N-terminal profragment in the ER,. The generated 87-kDa protein is targeted to the regulated secretory pathway where it is further shortened by removal of 135 amino acids of its C-terminal tail, resulting in the 66-kDa form. This C-terminal cleavage occurs at the dibasic Arg-Arg617�C618 site, possibly by an autocatalytic event and this tail has been proposed to play a role in sorting of PC1/3 to the regulated secretory pathway. The reported pH assays showed that both 87 and 66-kDa forms have higher activity in acidic environment and along the secretory pathway PC1/3 finds an environment that pH decreases from 6.7 to 5.5. It also known that in the secretory granules calcium concentration can raise up to milimolar concentrations, ; and the peptidase activities of 87 and 66-kDa forms of PC1/3 are increased in calcium ion concentrations range 1 to 20 mM. PC1/3 was reported to present complex enzymatic kinetics for the hydrolysis of substrates, and a lag phase in the initial 8 to 10 minutes followed by a linear phase with a constant velocity of hydrolysis was reported. The aim of the present paper was to explore the unusual lag phase observed in the time course of activity of mPC1/3 as showed in Figure 1 for hydrolysis of the commercially available fluorogenic substrate pERTKR-AMC. The 66-kDa mPC1/3 exhibits substrate concentration dependent hysteresis that is found in enzymes involved in metabolic pathways, as Oxysophocarpine earlier reviewed by Frieden. This is an unprecedented observation in peptidases, but is frequent in regulatory enzymes with physiological relevance where this hysteretic behavior has been related to slow rate conformational changes in response to variations in the ligand concentration. In the regulatory enzymes the conformation changes represent the rate-limiting steps of their catalytic activities similar to that observed with mPC1/3, the lag phase of which is on the order of minutes in contrast to catalytic steps that are in seconds. The lag phase parameter k varies between two limiting values with increasing substrate concentration, and this behavior indicates the existence of an equilibrium between active and inactive states of the enzyme. The slow transition of inactive to active mPC1/3 can be interpreted accordingly to the concept of hysteretic enzymes.