Ent-Kaurene Biosynthesis in Extracts of Helianthus annuus L. Seedlings.

Kaurene synthetase B activity (conversion of copalyl pyrophosphate to ent-kaurene) is readily detectable in crude cell-free extracts of 3- to 4-day old dark-grown sunflower (Helianthus annuus cv. Mammoth) seedlings, whereas little or no kaurene synthetase AB activity (conversion of geranylgeranyl pyrophosphate to ent-kaurene) can be found in these extracts under comparable assay conditions. A low amount of AB activity is evident only if an extensively dialyzed extract is used in low concentrations as the enzyme source. One factor which may contribute to the low apparent levels of AB activity is the presence of inhibitory factors in the crude sunflower extract since these extracts can be shown to act as a potent inhibitor of Marah macrocarpus endosperm kaurene synthetase AB activity. Heat treatment (100 degrees C) or dialysis of the sunflower extract reduces the amount of its inhibitory activity. Also, it was observed that low concentrations of extensively dialyzed sunflower extracts act to stimulate M. macrocarpus AB activity. There is no evidence for the presence of an inhibitory factor for M. macrocarpus kaurene synthetase B activity in sunflower extracts. However, there does appear to be present in the crude preparation of sunflower extract a dialyzable factor(s) that impedes its own B activity. There is little information to date on the nature of these inhibitory and stimulatory factors for kaurene synthetase activity or their possible roles in physiological regulation. The possible presence of such factors should be considered, however, when attempting to evaluate kaurene synthetase activities in extracts of vegetative plants.