Question

Which one of the following reducing substances decreases the antimicrobial activity of lactenin in milk?

• A. Glutathione
• B. Citrates
• C. Lysozymes
• D. Vitamin A

Hint:

Lactoperoxidase System Chemistry: Hypothiocyanite ($\text{OSCN}^-$) oxidizes bacterial -SH groups. REDUCING AGENTS (Glutathione, Cysteine) reverse this oxidation and destroy lactenin potency.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Lactoperoxidase-thiocyanate-hydrogen peroxide system chemistry: the active antimicrobial hypothiocyanite radical ($ ext{OSCN}^-$) oxidizes bacterial metabolic sulfhydryl groups (-SH); reducing agents like reduced glutathione (GSH) reduce hypothiocyanite back, destroying lactenin activity.
Key Formula or Approach:
\[ \text{Antimicrobial: } \text{OSCN}^- + 2\text{R-SH (Bacterial Enzymes)} \longrightarrow \text{R-S-S-R (Inactivated)} + \text{H}_2\text{O} + \text{SCN}^- \]
\[ \text{Inhibition by GSH: } \text{OSCN}^- + 2\text{GSH} \longrightarrow \text{GSSG} + \text{SCN}^- + \text{H}_2\text{O} \quad [\text{Abolishes Activity}] \]

Step 2: Detailed Explanation:
In milk biochemistry and natural antibacterial systems:
- Lactenins (Lactenin 1 L1 = Lactoperoxidase, Lactenin 2 L2 = Immunoglobulins) constitute the natural antibacterial defense of raw milk.
- The Lactoperoxidase system generates short-lived oxidizing intermediates, predominantly Hypothiocyanite ($\text{OSCN}^-$) and hypothiocyanous acid ($\text{HOSCN}$), which kill bacteria by oxidizing essential intracellular thiol/sulfhydryl (-SH) enzymes (such as hexokinase and glyceraldehyde-3-phosphate dehydrogenase).
- When Glutathione (GSH) (a potent biological reducing thiol tripeptide) or cysteine is present, it rapidly and preferentially donates electrons to reduce $\text{OSCN}^-$ back into inert thiocyanate ($\text{SCN}^-$), thereby neutralizing and decreasing the antimicrobial activity of lactenin in milk.

Step 3: Final Answer:
Thus, Glutathione decreases the antimicrobial activity of lactenin, matching option (A).