A Schistosoma mansoni fatty acid-binding protein, Sm14, is the potential basis of a dual-purpose anti-helminth vaccine.

Molecular cloning of components of protective antigenic preparations has suggested that related parasite fatty acid-binding proteins could form the basis of the protective immune crossreactivity between the parasitic trematode worms Fasciola hepatica and Schistosoma mansoni. Molecular models of the two parasite proteins showed that both molecules adopt the same basic three-dimensional structure, consisting of a barrel-shaped molecule formed by 10 antiparallel beta-pleated strands joined by short loops, and revealed the likely presence of crossreactive, discontinuous epitopes principally derived from amino acids in the C-terminal portions of the molecules. A recombinant form of the S. mansoni antigen, rSm14, protected outbred Swiss mice by up to 67% against challenge with S. mansoni cercariae in the absence of adjuvant and without provoking any observable autoimmune response. The same antigen also provided complete protection against challenge with F. hepatica metacercariae in the same animal model. The results suggest that it may be possible to produce a single vaccine that would be effective against at least two parasites, F. hepatica and S. mansoni, of veterinary and human importance, respectively.

A catalytic mechanism for the dual-specific phosphatases.

Dual-specific protein-tyrosine phosphatases have the common active-site sequence motif HCXXGXXRS(T). The role of the conserved hydroxyl was investigated by changing serine-131 to an alanine (S131A) in the dual-specific protein-tyrosine phosphatase VHR. The pH profile of the kcat/Km value for the S131A mutant is indistinguishable from that of the native enzyme. In contrast, the kcat value for S131A mutant is 100-fold lower than that for the native enzyme, and the shape of the pH profile was perturbed from bell-shaped in the native enzyme to a pH-independent curve over the pH range 4.5-9.0. This evidence, along with results from a previous study, suggests that the S131A mutation alters the rate-limiting step in the catalytic mechanism. Formation of a phosphoenzyme intermediate appears to be rate-limiting with the native enzyme, whereas in the S131A mutant breakdown of the intermediate is rate-limiting. This was confirmed by the appearance of a burst of p-nitrophenol formation when p-nitrophenyl phosphate rapidly reacted with the S131A enzyme in a stopped-flow spectrophotometer. Loss of this hydroxyl group at the active site dramatically diminished the ability of the enzyme to hydrolyze the thiol-phosphate intermediate without exerting any significant change in the steps leading to and including the formation of the intermediate. Consistent with rate-limiting intermediate formation in the native enzyme, the rate of burst in the S131A mutant was 1.5 s-1, which agrees well with the kcat value of 5 s-1 observed for native enzyme. The amplitude of the burst was stoichiometric with final enzyme concentration, and the slow linear rate (0.06 s-1) of p-nitrophenol formation after the burst was in agreement with the steady-state determined value of kcat (0.055 s-1).