A study of the rate of recycling of triose phosphates in heterotrophic Chenopodium rubrum cells, potato tubers, and maize endosperm.

We have investigated whether starch accumulation in heterotrophic cell-suspension cultures of Chenopodium rubrum L., developing potato (Solarium tuberosum L.) tubers or maize (Zea mays L.) endosperm involves import of triose phosphates or of hexose units into the plastid, and whether there is a rapid recycling of triose phosphates back to hexose phosphates in the cytosol of these tissues, (i) Cell suspensions, potato tubers or intact maize kernels were supplied with [1-(14)C] glucose or [6-(14)C]glucose. The glucosyl residues from starch were isolated and degraded by an enzymic procedure to determine how much radioactivity had been redistributed into the opposite half of the glucose molecule. Randomisation was incomplete, affecting only 18%-38% of the molecules in Chenopodium, 16%-26% of the molecules in potato, or 30% of the molecules in maize. It is concluded that the major route for starch synthesis involves import of hexose units, (ii) The glucosyl and fructosyl moieties of sucrose were isolated and degraded to determine the extent of recycling in the cytosol. There was significant randomisation, lying between 30%-40% in Chenopodium, 20%-26% in potato, and 8%-12% in maize. It is concluded that there is considerable recycling of triose phosphates in the cytosol. (iii). Sucrose from cells supplied with [1-(14)C]glucose was more randomised than sucrose from cells supplied with [6-(14)C]glucose. This is explained in terms of the oxidative pentose-phosphate pathway (iv). Equations are used to estimate the rate of recycling from triose phosphates to hexose phosphates in the cytosol. The estimated rate of recycling is considerably larger than the net glycolytic flux or the activity of the oxidative pentose-phosphate cycle.