Product Manual

Lactose Assay Kit
Catalog Number
MET-5001

100 assays

FOR RESEARCH USE ONLY
Not for use in diagnostic procedures

Introduction
Lactose is a disaccharide form of sugar made up of one glucose and one galactose monosaccharide. In
animals, millimolar levels of lactose are found in breast milk. In order for lactose to be absorbed into
the bloodstream, lactase (secreted from the small intestines) cleaves lactose into glucose and galactosemonosaccharides.
The inability to digest lactose is known as lactose intolerance, which arises from a reduction or loss inproduction of lactase, and is now more commonly known as “lactase deficiency”. In most mammals,lactase deficiency is a normal condition that occurs shortly after weaning from breast milk as theprimary food source. In humans, a significant population remains tolerant to lactose into adulthood dueto the continued production of lactase in the small intestines. Some estimates suggest that 75% of theworld’s adult population is lactase deficient. Since the severity of lactose maldigestion symptoms candepend on the amount of lactose consumed, it is important to quantify the relative amounts of lactose invarious dairy food sources.
Cell Biolabs’ Lactose Assay Kit is a simple fluorometric assay that measures the amount of total lactose
present in milk-based food products or biological samples (such as blood or urine from lactatinganimals) in a 96-well microtiter plate format. Each kit provides sufficient reagents to perform up to 100
assays, including blanks, lactose standards, endogenous controls*, and unknown samples. Sample
lactose concentrations are determined by comparison with a known lactose standard. The kit has adetection sensitivity limit of 10 µM lactose.

Assay Principle
Cell Biolabs’ Lactose Assay Kit measures total lactose within food or biological samples. First, lactase
cleaves lactose into glucose and galactose. Glucose is oxidized by glucose oxidase into D-gluconic
acid plus hydrogen peroxide. The hydrogen peroxide is then detected with a highly specificfluorometric probe. Horseradish peroxidase catalyzes the reaction between the probe and hydrogenperoxide, which bind in a 1:1 ratio. Samples are compared to a known concentration of lactose
standard within the 96-well microtiter plate format. Samples and standards are incubated for 45
minutes and then read with a standard 96-well fluorometric plate reader (Figure 1).
*Note: Endogenous levels of glucose can interfere with the assay. Therefore, an endogenous control
must be run for each sample to account for potential interference of these molecules.

Figure 1. Lactose assay principle.
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Related Products
1.
2.
3.
4.

STA-680: Glucose Assay Kit (Colorimetric)
STA-681: Glucose Assay Kit (Fluorometric)
MET-5012: Lactate Assay Kit (Colorimetric)
MET-5013: Lactate Assay Kit (Fluorometric)

Kit Components
1.
2.
3.
4.

Lactose Standard (Part No. 50011C): One 200 µL tube at 20 mM
10X Assay Buffer (Part No. 50012A): One 25 mL bottle
Fluorometric Probe (Part No. 50013C): One 50 µL amber vial.
HRP (Part No. 234402-T): One 10 μL tube at 100 U/mL
Note: One unit is defined as the amount of enzyme that will form 1.0 mg purpurogallin frompyrogallol in 20 seconds at pH 6.0 and 20ºC.
5. Lactase (Part No. 50014C): One 500 µL tube at 20U/mL
Note: One unit will hydrolyze 1.0 μmole of lactose per minute at pH 4.5 at 30°C.
6. Glucose Oxidase (Part No. 50015C): One 100 µL tube at 200U/mL
Note: One unit is defined as the amount of enzyme that will oxidize 1.0 micromole of beta-Dglucose to D-gluconic acid and hydrogen peroxide per minute at pH 5.1 at 35°C.

Materials Not Supplied
1. Deionized water
2. 1X PBS
3. Standard 96-well fluorescence black microtiter plate and/or black cell culture microplate

Storage
Store the 10X Assay Buffer at room temperature. Store all other components at -20ºC. The
Fluorometric Probe is light sensitive and must be protected accordingly. Avoid multiple freeze/thawcycles.

Preparation of Reagents••1X Assay Buffer: Dilute the 10X Assay Buffer 1:10 with deionized water for a 1X solution. Stir orvortex to homogeneity.
Reaction Mix: Prepare a Reaction Mix by diluting the Fluorometric Probe 1:100, HRP 1:500,
Lactase 1:10 and Glucose Oxidase 1:50 in 1X Assay Buffer. For example, add 10 μL Fluorometric
Probe stock solution, 2 μL HRP stock solution, 100 µL of Lactase and 20 μL of Glucose Oxidase to868 µL 1X Assay Buffer for a total of 1 mL. The above example Reaction Mix volume is enoughfor 20 assays. The Reaction Mix is stable for 1 day at 4ºC.
Note: Scale the described example up or down as needed and prepare only enough for immediate
use.
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•

Endogenous Control Mix: Prepare Endogenous Control Mix (no Lactase) by diluting theFluorometric Probe 1:100, HRP 1:500, and Glucose Oxidase 1:50 in 1X Assay Buffer. For example,add 10 μL Fluorometric Probe stock solution, 2 μL HRP stock solution, 20 μL of Glucose Oxidase,to 968 µL 1X Assay Buffer for a total of 1 mL. The above example is enough for 20 assays. TheEndogenous Control Mix is stable for 1 day at 4ºC.
Note: Scale the described example up or down as needed and prepare only enough for immediateuse.

Preparation of Samples•Cell culture supernatants: Cell culture media formulated with lactose or glucose should be avoided.
To remove insoluble particles, centrifuge at 10,000 rpm for 5 min. The cell conditioned media may
be assayed directly or diluted as necessary. Prepare the Lactose standard curve in non-conditioned
media without lactose or glucose.
Note: Maintain pH between 7 and 8 for optimal working conditions as the Fluorometric Probe isunstable at high pH (>8.5).

•Tissue lysates: Sonicate or homogenize tissue sample in cold PBS or 1X Assay Buffer andcentrifuge at 10000 x g for 10 minutes at 4°C. Perform dilutions in 1X Assay Buffer.
Cell lysates: Resuspend cells at 1-2 x 106 cells/mL in PBS or 1X Assay Buffer. Homogenize orsonicate the cells on ice. Centrifuge to remove debris. Cell lysates may be assayed undiluted or
diluted as necessary in 1X Assay Buffer.
Serum, plasma or urine: To remove insoluble particles, centrifuge at 10,000 rpm for 5 min. Dilutethe supernatant from 1:50 to 1:200 with 1X Assay Buffer just prior to performing the assay.

••

Notes:• All samples should be assayed immediately or stored at -80°C for up to 1-2 months. Run proper
controls as necessary. Optimal experimental conditions for samples must be determined by theinvestigator. Always run a standard curve with samples.
• Samples with NADH concentrations above 10 μM and glutathione concentrations above 50 μM
will oxidize the Fluorometric Probe and could result in erroneous readings. To minimize thisinterference, it is recommended that superoxide dismutase (SOD) be added to the reaction at a
final concentration of 40 U/mL (Votyakova and Reynolds, Ref. 5).
• Avoid samples containing DTT or β-mercaptoethanol since Resorufin is not stable in thepresence of thiols (above 10 μM).

Preparation of Standard Curve
1. Prepare fresh Lactose standards before use by diluting in 1X Assay Buffer. First, dilute the Lactose
Standard 20 mM stock solution 1:20 in 1X Assay Buffer to make a 1 mM Lactose Solution (e.g.,add 25 μL of the stock 20 mM Lactose Standard to 475 μL of 1X Assay Buffer). Vortex thoroughly.
2. Use the 1 mM Lactose Solution to prepare a series of the remaining Lactose standards according toTable 1.

4StandardTubes
1234
56781 mM Lactose Solution
(µL)
80
64
48
32
168
40

1X Assay Buffer(µL)
320336
352
368
384
392
396
400

Lactose
(µM)
200
160
120
80
40
20
100Lactose(mg/dL)
6.84
5.47
4.10
2.74
1.37
0.68
0.34
0Table 1. Preparation of Lactose Standards.

Assay Protocol
1. Prepare and mix all reagents thoroughly before use. Each sample, including unknowns andstandards, should be assayed in duplicate or triplicate.
Note: Each sample replicate requires two paired wells, one positive well and one backgroundcontrol well.
2. Add 50 µL of each standard into wells of a black microtiter plate suitable for a fluorescence plate
reader.
3. Add 50 µL of each unknown sample to each of two separate wells.
4. Add 50 µL of Reaction Mix to all standard wells and one half of the paired sample wells.
5. Add 50 µL of Endogenous Control Mix to the remaining paired sample wells.
6. Mix all well contents thoroughly and incubate for 45-60 minutes at 37ºC protected from light.
Note: This assay is continuous (not terminated) and therefore may be measured at multiple time
points to follow the reaction kinetics.
7. Read the plate with a fluorescence microplate reader equipped for excitation in the 530-570 nm
range and for emission in the 590-600 nm range.
8. For each unknown sample, subtract the RFU reading using the Endogenous Control Mix from theRFU reading using the Reaction Mix.
9. Calculate the concentration of lactose within samples by comparing the sample RFU to the standardcurve.

5Example of Results
The following figures demonstrate typical Lactose Assay Kit results. One should use the data below forreference only. This data should not be used to interpret or calculate actual sample results.

Figure 2: Lactose standard curve.

Figure 3: Lactose detection in bovine nonfat dry milk using the Lactose Assay Kit.
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References
1. Swallow DM (2003). Annual Review of Genetics 37: 197–219.
2. Ingram CJ, Mulcare CA, Itan Y, Thomas MG, Swallow DM (Jan 2009). Hum Genet 124 (6): 57991.
3. Lactose Tolerance and Milk Consumption. (1988) Am J Clin Nutr. Ch 3 Lactose content of milkand milk products, 48:1099-1104.
4. Argüelles AF, Rodríguez L, Tenias J, Otero M, Casellas Jordá F, Blay Cortés G, Lucendo Villarín
AJ, Domínguez Jiménez JL, Carballo Alvarez F, Aranoff SL, Berkowitz K, Shreiner B, and Want L(2015) Rev Esp Enferm Dig. 107.
5. Votyakova TV, and Reynolds IJ (2001) Neurochem. 79:266.

Recent Product Citation
1. Li, Y. et al. (2021). Nrf2-ARE Signaling Partially Attenuates Lipopolysaccharide-Induced
Mammary Lesions via Regulation of Oxidative and Organelle Stresses but Not Inflammatory
Response in Mice. Oxid Med Cell Longev. doi: 10.1155/2021/8821833.

Warranty
These products are warranted to perform as described in their labeling and in Cell Biolabs literature when used inaccordance with their instructions. THERE ARE NO WARRANTIES THAT EXTEND BEYOND THIS EXPRESSEDWARRANTY AND CELL BIOLABS DISCLAIMS ANY IMPLIED WARRANTY OF MERCHANTABILITY OR
WARRANTY OF FITNESS FOR PARTICULAR PURPOSE. CELL BIOLABS’s sole obligation and purchaser’s
exclusive remedy for breach of this warranty shall be, at the option of CELL BIOLABS, to repair or replace the products. In
no event shall CELL BIOLABS be liable for any proximate, incidental or consequential damages in connection with theproducts.

Contact Information
Cell Biolabs, Inc.
5628 Copley Drive
San Diego, CA 92111
Worldwide: +1 858 271-6500
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E-mail: tech@cellbiolabs.com
www.cellbiolabs.com©2015-2024: Cell Biolabs, Inc. - All rights reserved. No part of these works may be reproduced in any form without
permissions in writing.

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