A protein acting as a powerful inhibitor of plant pectin methylesterase was isolated from kiwi (Actinidia chinensis) fruit. The complete amino‐acid sequence of the pectin methylesterase inhibitor (PMEI) was determined by direct protein analysis. The sequence comprises 152 amino‐acid residues, accounting for a molecular mass of 16 277 Da. The far‐UV CD spectrum indicated a predominant alpha‐helix conformation in the secondary structure. The protein has five cysteine residues but neither tryptophan nor methionine. Analysis of fragments obtained after digestion of the protein alkylated without previous reduction identified two disulfide bridges connecting Cys9 with Cys18, and Cys74 with Cys114; Cys140 bears a free thiol group. A database search pointed out a similarity between PMEI and plant invertase inhibitors. In particular, the four Cys residues, which in PMEI are involved in the disulfide bridges, are conserved. This allows us to infer that also in the homologous proteins, whose primary structure was deduced only by cDNA sequencing, those cysteine residues are engaged in two disulfide bridges, and constitute a common structural motif. The comparison of the sequence of these inhibitors confirms the existence of a novel class of proteins with moderate but significant sequence conservation, comprising plant proteins acting as inhibitors of sugar metabolism enzymes, and probably involved in various steps of plant development.

Food allergy is increasingly common worldwide. Tools for allergy diagnosis measuring IgE improved much since allergenic molecules and microarrays started to be used. IgE response toward allergens belonging to the same group of molecules has not been comprehensively explored using such approach yet. Using the model of lipid transfer proteins (LTPs) from plants as allergens, including two new structures, we sought to define how heterogeneous is the behavior of homologous proteins. Two new allergenic LTPs, Act d 10 and Act c 10, have been identified in green (Actinidia deliciosa) and gold (Actinidia chinensis) kiwifruit (KF), respectively, using clinically characterized allergic patients, and their biochemical features comparatively evaluated by means of amino acid sequence alignments. Along with other five LTPs from peach, mulberry, hazelnut, peanut, mugwort, KF LTPs, preliminary tested positive for IgE, have been immobilized on a microarray, used for IgE testing 1,003 allergic subjects. Comparative analysis has been carried out. Alignment of Act d 10 primary structure with the other allergenic LTPs shows amino acid identities to be in a narrow range between 40 and 55%, with a number of substitutions making the sequences quite different from each other. Although peach LTP dominates the IgE immune response in terms of prevalence, epitope recognition driven by sequence heterogeneity has been recorded to be distributed in a wide range of behaviors. KF LTPs IgE positive results were obtained in a patient subset IgE positive for the peach LTP. Anyhow, the negative results on homologous molecules allowed us to reintroduce KF in patients' diet. The biochemical nature of allergenic molecule belonging to a group of homologous ones should not be taken as proof of immunological recognition as well. The availability of panels of homologous molecules to be tested using microarrays is valuable to address the therapeutic intervention.