Oxidative Enzymic Metabolism of 1-MCP by Tissues and Cell-Free Homogenates of Fruits and Vegetables

A number of fruit and vegetable tissues show high non-specific sorption of 1-MCP. In experiments comparing sorption of gaseous 1-MCP (SmartFresh^TM Technology, 765 µmol m^-3, 18.2 µL L^-1 ) to whole and processed apple fruit, sorption rate increased from 3.0 ± 0.2 ng kg^-1 s^-1 in intact fruit to 13.8 ± 2.4 and 28.2 ± 1.5 ng kg^-1 s^-1 in halved and fresh-cut wedges, respectively. Peeled fruit also showed sharply enhanced sorption, indicating that sorption was restricted by epidermal tissue and/or enhanced in response to tissue wounding. Sorption by fresh-cut apple was minimally affected by freeze/thaw-mediated tissue disruption (24%) but was strongly inhibited (90%) in response to heating.  The objective of this study was to determine if enzymic metabolism participates in tissue 1-MCP consumption. Analyses revealed that cell-free homogenates (CFH) from apple fruit metabolized 1-MCP at rates in excess of 100 ng kg^-1 s^-1. Activity was negligible in buffer-insoluble residues (cell walls). Typical 1-MCP metabolism assays utilized 10 mL CFH from 5.0 g of tissue along with 10 mL 125 mol m^-3 Na-MES, pH 5.0. The solutions were sealed in 244 mL jars and provided with 420.5 µmol m^-3 (10 µL L^-1) gaseous 1-MCP. 1-MCP  metabolism occurred optimally at pH 5, and was inhibited by heating (>90%), ascorbate (83% at 4 mol m^-3), Na-hypotaurine (75% at 1 mM), hypoxia (45% at 0.25 kPa O₂), sodium docecyl sulfate (63% at 34 mol m^-3), and was eliminated upon centrifugation of CFH 100,000 x g. 1-MCP metabolism displayed saturation kinetics, with a K_m of 160 mmol m^-3 and V_max of 4.12 µmol kg^-1 s^-1. Asparagus spears rank among the highest of fruits and vegetables in terms of 1-MCP sorption rate and capacity. As with apple tissue, sorption of 1-MCP to asparagus spears was strongly inhibited by heating.  By contrast with apple, however, 1-MCP sorption by asparagus was strongly inhibited by freeze thaw (50%) and 1-MCP metabolism was not detected in asparagus CFH.  Experiments employing combined CFH from apple and asparagus revealed that asparagus spears contained compounds capable of inhibiting 1-MCP metabolism in apple CFH (75%). 1-MCP metabolism was also detected in CFH from plantain peel but not pulp, consistent with the high and low sorption capacities of the respective tissues. The data suggest that membrane-associated enzymic oxidation rather than physical binding to cellular macromolecules constitutes the primary sink for 1-MCP applied to fruit and vegetable tissues.