Skip to content
Home » Lead Sulfide Test for Cysteine and Cystine

Lead Sulfide Test for Cysteine and Cystine

In this lead sulfide test for cysteine and cystine post we briefly summarise about: principle, reagents requirements, procedure, result, application and limitations of lead sulfide test.

Lead Sulfide Test for Cysteine and Cystine

Lead sulfide test (or Lead acetate test) is a biochemical test for the detection of amino acids like cysteine and cystine. The test is designed to detect sulfur-containing amino acids, the S-S group in cysteine, and the S-H group in cystine. 

The reagent for the test is lead acetate, hence it’s also known as a lead acetate test. Despite the fact that the test is designed to detect sulfur-containing amino acids, methionine does not produce a positive lead sulfide test result.

Objectives

To determine whether a sample contains sulfur-containing amino acids.

In order to detect cysteine-containing proteins and cystine in a sample.

To tell the difference between sulfur-containing and sulfur-free amino acids.

Principle

The test works by detecting sulfur in a solution by degrading the S-H or S-S group in amino acids under very alkaline conditions. In the presence of severe alkaline conditions at a high temperature, amino acids like cysteine and cystine release sulphur. 

After that, the sulphur reacts with the alkali (NaOH) to generate Na2. The resulting Na2S interacts with the lead acetate to create lead sulphide, leaving a dark residue. Free sulfur ions must be present in the media for the reaction to occur.

Reaction

Cysteine + 2NaOH  → Serine + Na2S + H2O

Pb(CH3COO)2 + NaOH  → Pb(OH)2 + 2CH3COONa

Pb(OH)2 + 2NaOH  → Pb(ONa)2 + 2H2O

Na2S + Pb(ONa)2 + H2O  → PbS (black precipitate) + 4NaOH

Requirements

Reagent

1. 2% lead acetate solution in water

2. 40% NaOH

3. Sample

Material

1. Test tubes

2. Test tube stand

3. Pipettes

Procedure

1. 2 mL of the amino acid solution is added to a test tube. 2 mL NaOH is added to this, and the solution is heated for one minute. A few drops of lead acetate are added to the solution once the test tube has cooled. The test tube is next examined to see if a precipitate has formed.

Result

Lead Sulfide Test

Lead Sulfide Test for Cysteine and Cystine

Positive test: The production of black precipitate at the bottom of the test tube indicates a positive result in the Lead sulphide test. This suggests that the solution contains cysteine or cystine.

Negative test: The absence of black residue in the test tube indicates a negative result in the Lead sulphide test. This means that cysteine or cystine is not present.

Uses

1. Sulfur-containing amino acids like cysteine and cystine are detected using this assay. And also this lead sulfide test aids in the differentiation of distinct amino acid groups.

2. The presence of cystine in urine is a pathological sign of disorders such as kidney and bladder cystine stones.

Limitations

1. The sulphur in the thioester bond in methionine is not liberated by the treatment with NaOH, hence it does not yield a positive result in this lead sulfide test.

2. If you add too much lead acetate to the solution in lead sulfide test, you could get white precipitation.

Further Readings

Reference

Join Our Telegram Channel!