Nebulizers effectiveness on pulmonary delivery of alpha-1 antitrypsin

The nebulization of alpha-1 antitrypsin (AAT) for its administration to the lung could be an interesting alternative to parenteral infusion for patients suffering from AAT genetic deficiency (AATD). In the case of protein therapeutics, the effect of the nebulization mode and rate on protein conformation and activity must be carefully considered. In this paper two types of nebulizers, i.e., a jet and a mesh vibrating system, were used to nebulize a commercial preparation of AAT for infusion and compared. The aerosolization performance, in terms of mass distribution, respirable fraction, and drug delivery efficiency, as well as the activity and aggregation state of AAT upon in vitro nebulization were investigated. The two nebulizers demonstrated equivalent aerosolization performances, but the mesh nebulizer provided a higher efficiency in the delivery of the dose. The activity of the protein was acceptably preserved by both nebulizers and no aggregation or changes in its conformation were identified. This suggests that nebulization of AAT represents a suitable administration strategy ready to be translated to the clinical practice for delivering the protein directly to the lungs in AATD patients, either as a support therapy to parenteral administration or for subjects with a precocious diagnosis, to prevent the onset of pulmonary symptoms.

Phosphoserine Aminotransferase Pathogenetic Variants in Serine Deficiency Disorders: A Functional Characterization

In humans, the phosphorylated pathway (PP) converts the glycolytic intermediate D-3-phosphoglycerate (3-PG) into L-serine through the enzymes 3-phosphoglycerate dehydrogenase, phosphoserine aminotransferase (PSAT) and phosphoserine phosphatase. From the pathogenic point of view, the PP in the brain is particularly relevant, as genetic defects of any of the three enzymes are associated with a group of neurometabolic disorders known as serine deficiency disorders (SDDs). We recombinantly expressed and characterized eight variants of PSAT associated with SDDs and two non-SDD associated variants. We show that the pathogenetic mechanisms in SDDs are extremely diverse, including low affinity of the cofactor pyridoxal 5'-phosphate and thermal instability for S179L and G79W PSAT, loss of activity of the holo form for R342W PSAT, aggregation for D100A PSAT, increased Km for one of the substrates with invariant kcats for S43R PSAT, and a combination of increased Km and decreased kcat for C245R PSAT. Finally, we show that the flux through the in vitro reconstructed PP at physiological concentrations of substrates and enzymes is extremely sensitive to alterations of the functional properties of PSAT variants, confirming PSAT dysfunctions as a cause of SSDs.

The allosteric interplay between S-nitrosylation and glycine binding controls the activity of human serine racemase

Human serine racemase (hSR) catalyzes the biosynthesis of D-serine, an obligatory co-agonist of the NMDA receptors. It was previously found that the reversible S-nitrosylation of Cys113 reduces hSR activity. Here, we show by site-directed mutagenesis, fluorescence spectroscopy, mass spectrometry, and molecular dynamics that S-nitrosylation stabilizes an open, less-active conformation of the enzyme. The reaction of hSR with either NO or nitroso donors is conformation-dependent and occurs only in the conformation stabilized by the allosteric effector ATP, in which the ε-amino group of Lys114 acts as a base toward the thiol group of Cys113. In the closed conformation stabilized by glycine-an active-site ligand of hSR-the side chain of Lys114 moves away from that of Cys113, while the carboxyl side-chain group of Asp318 moves significantly closer, increasing the thiol pK_a and preventing the reaction. We conclude that ATP binding, glycine binding, and S-nitrosylation constitute a three-way regulation mechanism for the tight control of hSR activity. We also show that Cys113 undergoes H₂ O₂ -mediated oxidation, with loss of enzyme activity, a reaction also dependent on hSR conformation.

Human serine racemase is inhibited by glyceraldehyde 3-phosphate, but not by glyceraldehyde 3-phosphate dehydrogenase

Murine serine racemase (SR), the enzyme responsible for the biosynthesis of the neuromodulator d-serine, was reported to form a complex with glyceraldehyde 3-phosphate dehydrogenase (GAPDH), resulting in SR inhibition. In this work, we investigated the interaction between the two human orthologues. We were not able to observe neither the inhibition nor the formation of the SR-GAPDH complex. Rather, hSR is inhibited by the hGAPDH substrate glyceraldehyde 3-phosphate (G3P) in a time- and concentration-dependent fashion, likely through a covalent reaction of the aldehyde functional group. The inhibition was similar for the two G3P enantiomers but it was not observed for structurally similar aldehydes. We ruled out a mechanism of inhibition based on the competition with either pyridoxal phosphate (PLP) - described for other PLP-dependent enzymes when incubated with small aldehydes - or ATP. Nevertheless, the inhibition time course was affected by the presence of hSR allosteric and orthosteric ligands, suggesting a conformation-dependence of the reaction.