Neutral invertase is a novel type of sucrose-cleaving enzyme

Metabolism of sucrose, a mobile source of energy and carbon, is an absolute requirement for the survival of heterotrophic plant organs, In these organs, different isoforms of invertase with discrete subcellular locations hydrolyze the disaccharide into hexoses and, thereby, feed sucrose into various biochemical pathways. In contrast to the invertases with acidic pH optima and a vacuolar or extracellular location, knowledge about the molecular nature of the cytoplasmic invertases (neutral and alkaline invertases) is still in its infancy, Here we report the cDNA cloning of a neutral invertase from carrot based on a partial peptide sequence from the purified enzyme. The function of the encoded polypeptide was confirmed by expression of the cDNA in Escherichia coli. The recombinant protein cleaved sucrose into glucose and fructose with the highest activity between pH 6.5 and 7.0. Unlike acid invertase, neutral invertase does not have the characteristics of a typical plant beta-fructofuranosidase and sucrose appears to be its sole substrate, The deduced amino acid sequence shares no similarity with sequences of acid invertases. The polypeptide is cysteine-rich and homologous sequences were only detected in the genomes of plants and photosynthetic bacteria, Hence, this protein must have evolved independently of other sucrose-cleaving enzymes, Transcripts for neutral invertase were found in all organs at different stages of development with slightly higher levels in developing organs, suggesting a more general and possibly a growth-related function of the enzyme in carrot sucrose metabolism.