Table 2.

Sulfonucleotide reductase activity of APR1

Reductant	Specific activity
	JM96		JM96-APR1		A522
	PAPS	APS	PAPS	APS	PAPS	APS
No DTT	0	0	1	5	0	0
+ DTT	0	1	7	222	0	0
+ DTT + Trx	0	0	4	226	320	10

Data are given by strain (JM96, JM96–APR1, and A522), and by sulfonucleotide (PAPS and APS). Reaction conditions: 100 μl volume, 50 mM Tris·HCl (pH 8.0), 1 mM EDTA, 25 mM NaF, 0.025 mM [³⁵S]PAPS or APS (57.7 Bq pmol⁻¹), and 0.02 or 0.04 mg cell protein. The following components, when added, were as follows: 5 mM DTT, 7.5 units P1 nuclease, and 0.01 mg Trx. Incubation was for 15 min at 30°C. Each value represents a rate calculated from two measurements with 0.02 and 0.04 mg cell protein. Specific activity = pmoles sulfite min⁻¹·mg⁻¹ protein. Zero activity = less than 1, the minimum, clearly measureable activity is ≈0.1 pmol sulfite formed.