Table 1.

Mammalian PLP-dependent enzymes with l-cysteine S-conjugate β-lyase activity^a

β-Lyase substrates	DCVC	TFEC	BTC	Syncatalytic inactivation	Competing transamination	Approximate specific activity (U/mgb)
Enzyme (cytosolic)
Kynureninase (R)	+	ND	+	+	ND	0.25
GTK/KAT I (R)c	+	+	−	−	+	0.6–6.4
GTK/KAT I (H)	+	+	+	−	+	8–40
GTL/KAT III (m)	+	+	−	−	+	0.5–1.1
cytAspAT (R)	+	+	±	+	−	0.04–0.16
AlaAT (P)	+	+	+	+	−	0.004–0.06
BCATc (H)	+	+	+	+	−	0.3–0.5
Enzyme (mitochondrial)
mitAspAT (R)	+	+	+	+	+	0.8–2.3
BCATm (H)	+	+	−	+	−	0.2–0.5
AGAT II (R)	+	+	+	+	+	0.2
GABA aminotransferase (P)	ND	+	ND	ND	ND	0.016
High-Mr β-lyase (R)d	+	+	+	−	+	1.0–1.2

^aThis table is an update of that of Cooper and Pinto, 2006, Cooper and Pinto, 2008 and Cooper et al. (2011). See also Pinto et al. (2014) and Cooper et al. (2016). A unit of enzyme activity (U) is defined as the amount of enzyme that catalyzes the formation of 1 μmol of pyruvate per min (usually at 37°C but temperature was not always specified). ND, not determined; R, rat; P, pig; H, human; and m, mouse

^bActivity with DCVC and/or TFEC.

^cHuman liver contains a KAT with strong cysteine S-conjugate β-lyase activity. A cysteine S-conjugate β-lyase has been highly purified from human kidney. The lyase activity copurifies with GTK. The human GTK, unlike the rat enzyme, has activity with BTC. Some GTK activity is also found in rat kidney and liver mitochondria, but the role of mitochondrial GTK as a cysteine S-conjugate β-lyase is uncertain.

^dActivity is also present in the cytosol.