Dark Metabolism of Carbon Monoxide in Lettuce Leaf Discs

Abstract

In the dark, leaf tissue of crisphead lettuce (Lactuca sativa L.) metabolized (14)CO to (14)CO(2) and acid-stable products. Tissue incubated at 2.5 degrees C for 3.5 hours and 48 hours converted about 1% and 17%, respectively, of the applied (14)CO to (14)CO(2), and incorporated about 0.04% and 0.6% of the (14)C in acid-stable products. Examination of soluble acid-stable products from (14)CO and (14)CO(2)-treated leaf tissue revealed that the labeling patterns of both treatments were identical during the 3.5-hour and the 48-hour incubation periods. Malate, citrate, and aspartate together comprised 70% or more of the soluble radioactivity from both treatments. Incorporation of radioactivity from CO into soluble acid-stable products during a 3-hour incubation period at 2.5 degrees C was inhibited 90% by adding 3% nonradioactive CO(2). These results indicate that in head lettuce in the dark (14)CO is metabolized primarily to (14)CO(2) which is the precursor of acid-stable products. In leaf discs at 2.5 degrees C, the apparent K(m) for CO oxidation to CO(2) was 5.3 microliters per liter and the V(max) was 9.7 nanoliters per gram per hour. The mitochondrial fraction of the leaf homogenate was the most active fraction to oxidize CO to CO(2), and this activity was heat-labile and cyanide-sensitive.