Lemna methionine synthesis

Most of the inorganic sulfate assimilated and reduced by plants appears ultimately in cysteine and methionine. These amino acids contain about 90% of the total sulfur in most plants (Allaway and Thompson, 1966). Nearly all of the cysteine and methionine is in protein. The typical dominance of protein cysteine and protein methionine in the total organic sulfur is illustrated in Table I by analyses of the sulfur components of a lower plant (Chlorella) and a higher plant (Lemna). Thede novo synthesis of cysteine and methionine is one of the key reactions in biology, comparable in importance to the reduction of carbon in photosynthesis (Allaway, 1970). This is so because all nonruminant animals studied require a dietary source of methionine or its precursor, homocysteine. Animals metabolize methionine via cysteine to inorganic sulfate. Plants complete the cycle of sulfur by reduction of inorganic sulfate back to cysteine and methionine, and are thus the ultimate source of the methionine in most animal diets (Siegel, 1975). 

Phosphohomoserine serves as a precursor of both threonine and methionine in higher plants, and regulation of its utilization in both branches of the pathway would be expected. This appears to occur, in part, by 5-adenosylmethio-nine activation of threonine synthase (5). Results obtained with partially purified Lemna threonine synthase (Giovanelli et al, 1984) indicate that the enzyme is essentially inactive in the absence of -adenosylmethionine, which cooperatively activates the enzyme at concentrations of less than 100 /iM. Conceptually, methionine could be synthesized and converted to S-adenosylmethionine prior to enzyme activation and the synthesis of threonine. Both orthophosphate and AMP inhibit Lemna threonine synthase in vitro, but the physiological significance of these effects is uncertain (Giovanelli et al, 1986). 

The results of these studies (Giovanelli et al, 1985b) are given for Lemna in Fig. 8. They show that the rate of synthesis of methionine from homocysteine is about five times faster than the rate of incorporation of sulfate sulfur into homocysteine and that metabolism of methionine to SAM is about four times more rapid than the rate of incorporation of methionine into the methionyl residues of protein. By contrast, an analysis of the products of p S]sulfate incorporation (Section II) shows that protein methionine is the single most important sink for the incorporation of inorganic sulfur. The answer to this... 

---