Aminoacid metabolism in plants : the biosynthesis of [beta]-cyanoalanine and asparagine in lupins : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Biochemistry at Massey University
β-Cyanoalanine synthase in etiolated seedlings of Lupinus
angustifolius is found mainly in the mitochondrial fraction of the cotyledons
and stems. In seedlings developing at 25°C in the dark it reaches a
maximal concentration after 5 days, at the same time as asparagine accumulation is most rapid and other physiological changes occur. However,
maximal ability to assimilate HCN gas to asparagine develops after 3 to 4
days, before asparagine accumulation begins.
A partial purification of lupin β-cyanoalanine synthase is described
and its distinction from cysteine synthase confirmed.
Carbon-14 labelled substrates were supplied to etiolated seedlings
of lupin in order to identify precursors of asparagine. Four carbon acids
related to the TCA cycle were readily converted to asparagine i n vivo
but not in vitro; the carbon skeleton of aspartate is retained in asparagine.
Comparison of the distribution of label in the carbon skeletons of aspartate
and asparagine from plants supplied carboxyl labelled fumarate shows a
separation of the aspartate pool for asparagine biosynthesis from most cell
aspartate. Metobolites that could be expected to give rise to cyanide and β-cyanoalanine are relatively ineffective as asparagine precursors.
Thus the β-cyanoalanine pathway is not of major importance in
asparagine biosynthesis in lupins.