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Maize seeds and five-day-old maize seedlings were incubated in media containing Pb 2+ at concentrations of 50, 100, and 200 mg 1 -l and Cd 2+ at concentrations of 1, 5, 10 and 50 mg 1 -l. After five days of incubation, both heavy metals were determined by means of AAS following wet mineralisation of roots and shoots. The results obtained indicate that Pb 2 § were transported to shoots from roots at a lower rate than Cd 2+.Phosphoenolpyruvate carboxylase (PEPC) isolated from germinating maize seeds was inhibited to a comparable degree by solutions containing 0.001 mmol 1-1 Pb 2+, 0.01 mmol 1 -I Cd 2+, and 0.005 mmol 1-1 Cu 2 § The enzyme was protected against this inhibition by the addition of mercaptoethanol, the substrate (PEP), or the cofactor (Mg 2 § The inhibition increased during a 20 min incubation of the enzyme with salts of the metals. Mn 2+, Ni 2+, and Co 2 § ions could partially substitute for the metal cofactor Mg 2 § Km values for these metal ions were as follows: for Mg 2 § 0.07 mmol 1 -l in the range from 0 to 0.30 mmol 1-1 Mg2+; 0.71 mmol 1 -I for 0.30 to 2.50 mmol 1 -~ MgZ+; for Mn ~ § 0.36 mmol l-l; for Ni 2 § 0.34 mmol l-l; and for Co 2+ 0.20 mmol 1 -l. The activity of the enzyme reached with Mn 2+ 85 %, with Ni 2+ 65 %, and with Co 2 § 55 % of the activity recorded with Mg 2+.

Phosphoenolpyruvate carboxylase (PEPC) from maize leaves has an M r of 400000. The native enzyme molecule is a homotetramer. The amino acid composition of PEPC is determined. The enzyme contains 8 half‐cystine residues per subunit. The role of half‐cystine residues and the steric arrangement of the enzyme protein molecule are discussed.

Pyruvate decarboxylase (EC 4.1.1.1), isolated from 4-day-old germinating peas, was precipitated from a sodium phosphate extract when (NH4)2SO4 was increased from 15 to 30% saturation, desalted on Sephadex G-25 or by dialysis for 24 h, and then chromatographed on a DEAE-cellulose column. This procedure increased the specific activity of the enzyme 120-fold compared with the sodium phosphate extract. The behaviour of the enzyme during gel filtration indicates that it has a high molecular weight. The pea enzyme exhibits a sigmoid dependence on the pyruvate concentration; reaction velocity is half-maximal at a substrate concentration of 1.8 mM and the Hill coefficient is 1.8. Thiamin pyrophosphate (TPP) is the coenzyme, which is relatively firmly bound to the apoenzyme, but can be removed by dialysis for 48 h. The apoenzyme is activated optimally at 2 mM TPP and inhibited by concentrations above 4 mM. The pH optimum for pea pyruvate decarboxylase is 5.8 and maximal temperature stability occurs at 48°C.