Unit 6: Enzyme Activity

I. Orientation

Enzymes are biological catalysts that increase the rate of specific chemical reactions without being permanently consumed. Salivary amylase, also called ptyalin, begins carbohydrate digestion in the mouth by hydrolyzing internal α-1,4-glycosidic bonds in starch. In the laboratory, its activity is determined by incubating saliva with a measured starch substrate and estimating the starch remaining after a fixed time, commonly through the iodine–starch color reaction.

• Enzyme activity: The rate at which an enzyme converts substrate into product under specified conditions; it is commonly expressed as amount of substrate consumed or product formed per unit time.
• Substrate: The reactant on which an enzyme acts. In this experiment, the principal substrate is soluble starch.
• Product: The compounds formed during catalysis. Salivary amylase produces shorter dextrins, maltose, and other reducing oligosaccharides.
• Specificity: Amylase acts mainly on starch and related α-1,4-linked polysaccharides; it does not hydrolyze every carbohydrate.
• Iodine indicator principle: Iodine forms a blue-black complex with intact starch. As starch is hydrolyzed, the color becomes weaker, reddish-brown, or absent.
• Fixed experimental conditions: Enzyme concentration, starch concentration, pH, temperature, incubation time, and sample volume must be controlled when comparing tubes.
• Initial-rate assumption: During a short incubation, substrate concentration is sufficiently high and product accumulation is limited, so the measured change approximates enzyme activity.
• Control requirement: A blank or control lacking active enzyme identifies color caused by reagents or substrate rather than enzymatic hydrolysis.
• Temperature dependence: Activity rises with temperature up to an optimum, but excessive heat denatures the protein. Human salivary amylase is usually most active near physiological temperature, approximately 37°C.
• pH dependence: Amylase activity depends on ionization of catalytic groups; salivary amylase generally functions best near neutral pH, commonly around pH 6.7–7.0.

II. Determination of salivary amylase enzyme activity — iodine method

A. Purpose and principle

The purpose of this method is to estimate salivary amylase activity by measuring the disappearance of starch after a controlled incubation. The procedure is a comparative colorimetric assay: equal starch aliquots are treated with saliva for a known period, and iodine is then added to reveal the amount of starch remaining.

• Reaction basis: Amylase catalyzes hydrolysis of starch in the presence of water.

Starch + H2O  --salivary amylase-->  dextrins + maltose + shorter oligosaccharides

• Color basis: Intact starch produces a deep blue-black color with iodine–potassium iodide reagent; hydrolyzed starch produces progressively less intense color.
• Activity interpretation: A paler iodine color indicates less residual starch and therefore greater amylase activity, provided the incubation time and starting starch amount are identical.
• Operational definition: One laboratory unit may be defined by the amount of enzyme that hydrolyzes a stated amount of starch under specified conditions in one minute. If no formal unit is supplied, results should be reported comparatively, such as relative activity or dilution endpoint.
• Essential condition: The saliva and starch must be mixed rapidly and incubated for exactly the same period for every test tube.

B. Determination of salivary amylase enzyme activity

The determination involves preparing a saliva sample, exposing starch to the enzyme under controlled conditions, stopping or visualizing the reaction with iodine, and comparing the resulting color with controls or standards.

• Materials: Typical materials include fresh saliva, soluble starch solution, phosphate buffer or another near-neutral buffer, iodine–potassium iodide solution, test tubes, pipettes, a water bath, stopwatch, and a white background or colorimeter.
  • Starch solution: A known concentration, such as 1%, supplies a reproducible substrate.
  • Buffer: Maintains pH during incubation because enzyme activity changes when pH drifts.
  • Iodine reagent: Usually contains iodine and potassium iodide; it is an indicator, not the hydrolyzing enzyme.
• Saliva collection: Rinse the mouth with water, wait briefly, and collect unstimulated or stimulated saliva according to the laboratory protocol.
  • Standardization: Record whether saliva was diluted, the dilution factor, and the volume used.
  • Reason for dilution: Undiluted saliva may digest starch too rapidly to distinguish differences during a short assay.
• Suggested tube arrangement: Label a test tube for the enzyme reaction and another for the blank.

• Pre-incubation: Bring starch, buffer, and saliva separately to the selected temperature, such as 37°C, before mixing.
  • Reason: Temperature differences at mixing can create unequal reaction rates.
  • Timing point: Define time zero as the instant saliva contacts starch.
• Incubation: Mix the test tube thoroughly and incubate for a measured interval, for example 5–10 minutes at 37°C.
  • Uniformity: Use the same volume, temperature, mixing method, and time for all samples.
  • Avoiding extremes: Do not boil the active sample; high heat denatures amylase and produces an inactive control.
• Iodine testing: After incubation, place equal drops or volumes of each reaction mixture onto a white tile or into separate wells, then add an equal amount of iodine reagent.
  • Blue-black: Substantial starch remains.
  • Lighter blue, brown, or yellow-brown: Starch has been partly or nearly completely hydrolyzed.
• Qualitative endpoint: For serially diluted saliva, the highest dilution that gives no blue color with starch is called the endpoint dilution.
  • Interpretation: A higher endpoint dilution indicates greater enzyme activity because less enzyme concentration was sufficient to digest the starch.
  • Required comparison: The control must remain blue-black; otherwise the substrate, reagent, or timing may be unsuitable.
• Semi-quantitative result: Rank samples by color intensity or determine the endpoint dilution rather than assigning an absolute concentration.
• Quantitative color measurement: If a colorimeter is available, measure absorbance after iodine addition at an appropriate wavelength selected for the blue starch–iodine complex.
  • Absorbance relationship: Within a suitable range, greater residual starch gives greater absorbance.
  • Activity relationship: Enzyme activity is inversely related to residual-starch absorbance when initial starch and reaction time are fixed.

A simple comparative calculation can be made when residual starch is estimated from absorbance:

Relative activity ∝ (A_control − A_sample) / t

• A_control: Absorbance of the control containing starch but no active amylase.
• A_sample: Absorbance of the saliva-treated reaction.
• t: Incubation time, usually expressed in minutes.
• Meaning: A larger decrease in absorbance represents more starch hydrolysis during the same time interval. This is a relative activity value unless a calibrated standard curve and formal activity definition are provided.

C. Variables affecting the assay

The reliability of the determination depends on controlling variables that alter enzyme–substrate collisions, protein structure, or the iodine color reaction.

• Enzyme concentration: Increasing saliva volume or decreasing its dilution generally increases the reaction rate until substrate becomes limiting.
  • Prediction: A concentrated saliva sample should produce a paler iodine color than a more dilute sample after the same incubation.
• Substrate concentration: More starch can increase the rate initially, but at sufficiently high concentration the enzyme becomes saturated and the rate approaches a maximum.
  • Symbolic relationship: The Michaelis–Menten model describes this behavior:

v = (Vmax[S]) / (Km + [S])

• v: Initial reaction velocity.
• Vmax: Maximum velocity when enzyme active sites are effectively saturated.
• [S]: Substrate concentration.
• Km: Substrate concentration giving half of Vmax.
  • Temperature: Raising temperature toward the enzyme’s optimum usually increases activity; temperatures above the optimum can unfold amylase and reduce activity.
• Practical comparison: A 37°C sample should generally digest starch more rapidly than a cold sample, whereas boiling saliva should show little or no activity.
  • pH: A buffer near pH 7 supports salivary amylase better than strongly acidic or alkaline conditions.
• Mechanism: Extreme pH changes the charge of amino acid side chains at or near the active site and may disrupt enzyme structure.
  • Incubation time: Longer incubation allows more starch hydrolysis, but the rate may slow as starch is depleted or products accumulate.
• Timing error: Starting the stopwatch late for one tube falsely lowers its calculated activity.
  • Mixing and volume: Unequal mixing, evaporation, pipetting error, or different iodine volumes changes color independently of enzyme action.
  • Saliva composition: Food residues, oral bacteria, mucus, recent eating, hydration, and individual variation can affect the measured activity.

D. Data interpretation and limitations

The result represents amylase performance under the exact laboratory conditions, not a permanent measure of a person’s overall digestive capacity.

• Color comparison: Interpret iodine colors against the control rather than relying only on an isolated visual shade.
  • Control valid: A strong blue-black control confirms that the starting starch reacts with iodine.
  • Test paler: Indicates enzyme-dependent loss of intact starch.
• Endpoint interpretation: If saliva dilutions of 1:2, 1:4, 1:8, and 1:16 are tested, an endpoint at 1:8 means the 1:8 dilution still hydrolyzed detectable starch, while the next dilution did not, according to the defined endpoint criterion.
• Replicate measurements: Duplicate or triplicate tubes improve reliability by revealing random pipetting or timing variation.
• Controls: A boiled-saliva control distinguishes enzymatic activity from nonenzymatic starch changes; a reagent blank detects background color.
• Major limitations: Visual iodine scoring is subjective, iodine color depends on starch structure and reagent concentration, and the method measures residual starch indirectly rather than product formation directly.
• Assay limitation: Salivary amylase may continue acting briefly after sampling unless the reaction is rapidly diluted, cooled, or otherwise stopped according to the protocol.
• Safety: Treat saliva as a biological specimen, wear gloves and eye protection, avoid mouth pipetting, disinfect work surfaces, and dispose of saliva-contaminated materials as instructed.
• Reporting: State the saliva dilution, starch concentration, pH, temperature, incubation time, iodine method, control result, endpoint or absorbance, and the interpretation of relative activity.
• Scientific conclusion: The strongest conclusion is conditional—for example, “Under the stated pH, temperature, substrate concentration, and incubation time, sample A hydrolyzed more starch than sample B.”