Persulfide generated from L-cysteine inactivates tyrosine aminotransferase. Requirement for a protein with cysteine oxidase activity and gamma-cystathionase.

J L Hargrove

doi:10.1016/s0021-9258(19)77830-2

Abstract

Liver cytosols contain factors that produce an inhibitor of tyrosine aminotransferase and other enzymes when incubated with L-cysteine or L-cystine. Cystine-dependent inactivation was caused by cystathionase and required pyridoxal 5'-phosphate, but a second protein was needed to reconstitute cysteine-dependent inactivation. A cytosolic protein was isolated that oxidized free cysteine and brought about inactivation of tyrosine aminotransferase when coincubated with cystathionase. Hematin also oxidized cysteine, which led to cysteine-dependent inactivation of tyrosine aminotransferase in the presence of cystathionase. The inactivation of tyrosine aminotransferase involved three steps: initial oxidation of cysteine to form cystine; desulfuration of cystine catalyzed by cystathionase to form the persulfide, thiocysteine; and reaction of thiocysteine (or products of its decomposition) with proteins to form protein-bound sulfane. Since dithiothreitol reactivated tyrosine aminotransferase, the sulfane probably inactivated the enzyme by oxidation of thiol groups. The present results do not indicate whether the cysteine oxidase activity is enzymatic nor do they prove which form of polysulfide inactivates tyrosine aminotransferase. Reduced glutathione greatly slowed the rates at which sulfane accumulated and at which tyrosine aminotransferase was inactivated. Incubation of DL-cystathionine with liver cytosols led to formation of cysteine, which was oxidized and cleaved to form persulfide, and caused inactivation of tyrosine aminotransferase. Thus, sulfane sulfur that is generated by an enzyme of the transulfuration pathway inactivates a transaminase by nonselective oxidation of enzyme-bound thiol groups.

Highlights

Liver cytosols contain factors thaptroduce an inhib- planation cannot account for cysteine-dependent enzyme initor of tyrosine aminotransferase and other enzymes activation, because some enzymes that do not rewhen incubated withL-cysteine or L-cystine
Cystine- quire pyridoxal 5”phosphate (PLP) are inactivated, and because inactivation does not dependent inactivation was caused by cystathionase occur in the absence of a factor from liver fractions
Hematin oxidized cysteine, which led cently identified asa second PLP-dependent enzyme, yto cysteine-dependent inactivation of tyrosine amino- cystathionase [8].This enzyme catalyzes the desulfuration of transferase in the presence of cystathionase

Summary

Persulfide and Enzyme Inactivation

High-speed (12,000X g,20min) supernatant fractions that inactivated tyrosine aminotransferase in the presence of both L-cysteine and L-cystine were prepared from livers of fasted rats [8].The capacity to inactivate tyrosine aminotransferase with either form of the amino acid was associated with proteins that remained soluble after acidification, were stable to heating, and could be precipitated with ammonium sulfate. The material that did not adsorb to DEAE-cellulose contained cystathionase and inactivated tyrosine aminotransferase after addition of Lcystine, but not with L-cysteine (line 6). The adsorbed material did not affect tyrosine aminotransferase (line 5) but restored cysteine-dependent inactivation when recombined with the unbound fraction (line 7)

Tyrosine aminotransferase inactivated

Findings

Persulfide andEnzyme Inactivation