Table I.

Effect of PP2A-mediated p107 dephosphorylation on kinetic constants of PEPC1 and PEPC2 from stage VII developing COS

Purified PEPC1 and PEPC2 were incubated for 20 min with 5 milliunits/mL of PP2A in the standard dephosphorylation assay buffer. Assays were conducted at pH 7.3. I₅₀ and K_a values were determined with subsaturating PEP (0.6 and 0.2 mm for PEPC1 and PEPC2, respectively). All values represent the mean of at least three separate experiments and are reproducible to within ±10% SE of the mean value.

Kinetic Parameter	PEPC1		PEPC2
	Phospho	Dephospho	Phospho	Dephospho
mm
Km(PEP)	0.55	0.92	0.28	0.18
I50(malate)	0.075	0.029	0.57	1.47
I50(glutamate)	2.1	2.2	4.1	7.0
I50(aspartate)	0.35	0.32	2.6	4.5
Ka(Glc-6-P)	0.22 (2.0)a	0.25 (1.5)	n.d.b	n.d.

^aValues in parenthesis indicate maximal fold activation of PEPC1 by saturating Glc-6-P.

^bn.d., Not determined (activity of phosphorylated [phospho] or dephosphorylated [dephospho] PEPC2 is essentially unresponsive to Glc-6-P).