Leucine Aminopeptidase Test: Principle, Procedure, and Result

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Leucine Aminopeptidase Test Introduction

Many of the biochemical test is usually used to detect the characteristics of the micro-organism especially to detect their activity when exposed or left in a medium of certain enzymes.

Mostly the biochemical tests are followed for bacteria which acts as a pathogen in the hosts organism.

Leucine Amino peptidase test is one among the biochemical test, which is commonly referred to as LAP test.

This test is followed by using the serum of the host. This test is usually used to detect the presence of an enzyme leucine amino peptidase for analyzing the preliminary characteristic of a catalase negative, a gram-positive coccus.

This is especially for a non-hemolytic coccus. Leucine Aminopeptidase is one of the cytosolic metallo-exopeptidase which helps in catalyzing the removal of the amino acids from the peptide, that is generated during the process of hemoglobin degradation.

What is Leucine Aminopeptidase Test?

Leucine Aminopeptidase Test is usually used o detect the activity of the catalase-negative gram-positive cocci.

This test specially differentiates the species Aerococcus and the Leuconoctoc from the other species like Streptococcus, Enterococcus, Lactococcus and Pediococcus.

Generally, it is one the rapid test followed for detecting the enzyme Leucine aminopeptidase.

Here, Leucine-Beta-naphthylamide impregnated disk acts as a substrate for detecting leucine aminopeptidase.

After hydrolyzing activity of the substrate by enzyme, it results the Beta-naphthylamine to produces a red color while adding the cinnamaldehyde reagent.

Leucine Aminopeptidase Test, is usually performed with the other jests in the form of conjugation experiments, for identifying the species of Streptococci and the other catalase negative gram-positive cocci.

Leucine Aminopeptidase Test Objective

The main aim of the test is to perform a preliminary characterization of the catalase negative, gram positive cocci.

Leucine Aminopeptidase Test Principle

Leucine aminopeptidase test is one of the rapid tests, used for detecting the presence of an enzyme leucine amino peptidase.

Here, the disks are impregnated with Leucine-Beta Naphthylamide, that is hydrolyzed by the enzyme known as leucine aminopeptidase, which is produced by the LAP positive organism.

This enzymatic activity thus results in the release of a substrate known as Beta-naphthylamine, Beta-naphthylamine, combines with the p-dimethylaminocinnamaldehyde reagent, at the time of adding it to the culture and results in the formation of highly visible red Schiff base.

Leucine Aminopeptidase Test Procedure

Initially, a Leucine Aminopeptidase disk is placed in a sterile petri disk in an aseptic condition and it is allowed to warm as of the room temperature.

Then it is slightly dampening the Leucine Aminopeptidase disk with the reagent grade water or with a small quantity of the sterile distilled water.

Then a small amount of several colonies of an 18-to-24-hour culture is applied on a small area of the Leucine Aminopeptidase disk using a wooden applicator stick.

Then the prepared medium is incubated at a room temperature for about 5 minutes.

After incubation the medium is taken out from the incubator and one drop of cinnamaldehyde reagent is added and the changes are noted within a minute.

Leucine Aminopeptidase Test Result

Positive Test: In case of positive result, development of red or pink color in the medium can be observed. For positive control presence of Enterococcus faecalis is determined.

Negative Test: In case of negative result, there will be no change in color. In few cases, a slight yellow color can be noted. For negative control Aero coccus viridians can be determined.

Leucine Aminopeptidase Test Uses

 LAP test is usually used in combination with the other tests like PYR and some biochemical tests. This helps us to differentiate between the catalase, gram positive cocci.

 Generally, the organisms like Streptococcus pneumoniae and Streptococcus pyogenes, Pedi coccus, Lactococcus and Enterococcus species are all considered and gives a positive result for LAP test.

 Where as the other species like Beta-hemolytic Streptococci, Aero coccus and Leuconostoc species are all considered as Negative control, and thus helps us in identifying the specific species.

 Aminopeptidase test have been used in performing a Gram analysis of the bacteria that is isolated from the Scleroderma citrinum mycorrhizae, the mycorrhizae and the bulk soil.

Leucine Aminopeptidase Test

 The test organism is confirmed as Gram-positive coccus and catalase negative before initiating the Leucine aminopeptidase test.

 Adequate inoculum is ensured, or else there will be many chances for getting negative results.

 Streptococci is usually tested prior to 48 hours of incubation or it is sub cultures before testing.

Clinical Uses Leucine Aminopeptidase Test

This test is performed in a clinical laboratory, using a blood sample. For collecting a blood, a specific person should have a fasting for about 8 hours.

Leucine Aminopeptidase is one of the types of protein and it is also known as enzyme which is normally found in the cells of the liver, blood and bile, placenta and urine.

This test is often suggested by the diagnostics when there is abnormal functioning of the liver or any damage is detected in the liver or any tumor in the liver.

Leucine Aminopeptidase Test Citations


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