Studies ofribulose-1,5-bisphosphate (RuBP) carboxylase from taxonomically diverse plants show that the enzyme from Cs and crassuacean acid metabolism pathway species exhibits lower Km(CO2) values (12-25 micromolar) than does that from C4 species (28-34 micromolar ground in a mortar with 100 mm Bicine buffer (pH 8.0) containing 25 mm MgCl2 and 1 mm DTT (about 4-10 ml). The homogenate was centrifuged at 25,000g for 15 min, and 0.5 ml of the supernatant was eluted through a 0.8 x 15 cm column of Sephadex G-25, equilibrated in the same buffer. The void volume which contained RuBP carboxylase was collected and saved. The partially purified enzyme extract (1.0 ml) was preactivated in 5 mm NaHCO3, and then assayed by measuring the fixation of ["C]-bicarbonate. The reaction mixture containing 100 mm Bicine and 25 mm MgCl2 (pH 8.0) was prepared C02-free, and flushed with N2 prior to using it. Assays (total volume of400 ,ul) were performed in l-ml stoppered vials (Pierce Reacti-vials No. 13221) which had been flushed with N2. For Km(CO2) determination, the HCO3 concentration ranged from 0.4 to 16.5 mM, with RuBP fixed at 0.5 mM; and for Km(RuBP) determination, the RuBP concentration ranged from 7.5 to 500,iM, with HCO3 fixed at 10.6 mm. Reaction was started by injection of fully activated enzyme (5 ,ld for Km [COu] assays; 2-40 ,ul for Km[RuBP] assays), and stopped after 1 min at 25 C by injection of 0.2 ml 2 N HCOOH. The reaction mixture was then quantitatively transferred to a glass scintillation vial and evaporated to dryness on a hot plate. After the vial had cooled, 1.0 ml distilled H20 was added, followed by a 9.0-ml mixture of 5 g PPO in 1 liter toluene plus 500 ml Triton X-100. Each vial was then counted for 5 min in a Searle Delta 300 liquid scintillation counter. The bicarbonate introduced into the assay solution with the enzyme aliquot was taken into consideration when calculating HCO3 concentration and specific radioactivity. The CO2 concentration was calculated from the pH and HCO3 concentration using the Henderson-Hasselbach equation and pK' value of 6.37 at 25 C (16). The Km values were statistically calculated using

No abstract

Table I, where the sample of grasses is arranged according to the best available information on taxonomic relationships above generic level, regarding both the contents of the major groupings and tribes, and (in so far as this can be achieved in a linear arrangement) the sequence of their presentation (16,28

Root protein contents of 15 cassava varieties (Manihot esculentaCrantz) ranged from 5 to 19 g kg−1 dry matter. Intervarietal differences in amino acid profiles of cassava roots were evident. Differences in the levels of aspartic acid, glutamic acid and arginine were most notable. The nitrogen‐to‐protein conversion factors (kAA) based on nitrogen recovered from total amino acid analyses including ammonia ranged from 4·75 to 5·87, showing that the traditional conversion factor of 6·25 was not valid for cassava root proteins. Conversion factors (kP) for 15 cassava varieties based on Kjeldahl nitrogen ranged from 2·49 to 3·67. Therefore an averagekPvalue of 3·24±0·31 may provide a better estimate of the protein content in cassava roots.

No abstract