649 



N. E. Tolbert 



Magnitude of the Glycolate Pathway 



To obtain a picture of the quantitative significance of the 

 pathway, amounts of C-'^'* in the reservoirs of phosphoglycolate, 

 glycolate, glycine and serine should be considered together. The 

 glycolate pathway is blocked by a-hydroxysulf onates which inhibit 

 glycolate oxidase (2) and by isonicotinyl hydrazide, an inhibitor 

 of transaminase (3). When either inhibitor is used with in vivo 

 photosynthesis a remarkable accumulation of C''-'* in glycolate 

 products results, for over 507o of the fixed C^'*02 in 10 minutes 

 may be present in glycolate and glycine. According to Zelitch 

 and Whittingham this accumulation occurs without a decrease in 

 the rate of CO2 fixation. However, Asada and Kasai (4) observed 

 over 707o inhibition of photosynthesis by 0.01 M a-hydroxysulf on- 

 ates. Such results suggest that much of the C-"^^ newly fixed 

 during photosynthesis is metabolized by the glycolate pathway. 



Even without inhibitors a very large proportion of the fixed 

 CO2 is found in glycolate, Warburg and Krippahl (5) reported 

 that Chorella during a 10 min. period converted 92% of the fixed 

 CO2 into glycolate-. We have not achieved such yields with Chlor- 

 ella, but have approached such values with Chlamydomonas , Also, 

 we can treat chloroplasts so that 757o of the fixed C-'-'^Oa will 

 appear in glycolate. Experiments were reproducible from day to 

 day with Chlamydomonas, but over periods of months values were 

 obtained which ranged from 25 to 75% of the total C-'-'* which ac- 

 cumulated in glycolate products. Many plant materials produce 

 much less glycolate products, but they all produce significant 

 amounts in brief periods of photosynthesis. It should be empha- 

 sized that glycolate accumulation occurs in particularly large 

 amounts with young tissue or with algae in the log phase of 

 growth. Algae of mixed ages can give misleading results. Older 

 tissue or tissues partially inhibited by water deficits and mis- 

 treatment will incorporate much of their C^^ into sucrose. With 

 care and with young tissue 50% of the C''^ fixed during a 10- 

 minute photosynthetic period in air will be found in glycolate 

 products when one uses wheat, barley, sugarbeets, Ankistrodesmus , 

 Chlamydomonas and, sometimes, Chlorella, 



Distribution of C-*-"^ in Components 



Calvin's group showed that glycolate produced during photosyn- 

 thesis had the same specific activity in both carbon atoms (6). 

 Thus all subsequent products of the glycolate pathway should be 

 uniformly labeled. We found that serine after 4 seconds of photo- 

 sjmthesis was uniformly labeled (1) even though the phosphogly- 

 cerate from the same experiments had 20 times more C^"^ in the car- 

 boxyl carbon than in the a and p carbons. Therefore the serine 



