Homocysteine Biosynthesis in Green Plants: Physiological Importance of the Transsulfuration Pathway in Lemna paucicostata

To permit an assessment of the relative contributions of the transsulfuration and the direct sulfhydration pathways for homocysteine biosynthesis, the time course of incorporation of ³⁵S from ³⁵SO₄²⁻ into various sulfur-containing compounds in Lemna paucicostata has been determined. Plants were grown with either low (4.5 micromolar) or ample (1,000 micromolar) sulfate in the medium. At the shortest labeling times, ³⁵S-cystathionine was the predominant ³⁵S-containing organic sulfur compound. The flux of sulfur into cystathionine was sufficient to sustain the known rate of methionine biosynthesis. It was calculated that transsulfuration accounted for at least 90 and 85% of the total homocysteine synthesis in low and ample sulfate-grown plants, respectively (and may have accounted for 100%). No marked rise in the ³⁵S-soluble cysteine:³⁵S-homocysteine ratio was observed even at the shortest labeling times, but it is argued that this may be due to (a) the observed compartmentation of soluble cysteine, and (b) the impracticality of using labeling times shorter than 17 seconds. Additional evidence supporting the importance of transsulfuration in Lemna is briefly described.