The situation of constitutive expression that can be enforced by induction is observed with many enzymes belonging to the cytochrome P450 superfamily and. is well understood in most cases.

For example, we detected CYP24A1 expression in different cell cultures that were essentially devoid of the natural substrates for extended time. After supplementing vitamin D metabolites, the expression of CYP24A1 rose up to 10 000 fold within few hours and declined rapidly to basal levels after having metabolized the substrates.

There are many reasons for increased enzyme activity beyond induced expression. Increased substrate could activate enzyme A by binding at an allosteric site or by indirectly by binding to an inhibitory factor. What is the time-frame for changes in A? Does the activity of B mirror its protein appearance? Also, you do not give details about your assays - could the activation be a Km change? Look at the kinetics to see if that provides a logical answer.

First, let's define the terms, since the choice depends heavily on how you define the two types of enzyme. I have done a quick google search and found these:

Constitutive enzyme:

1) An enzyme that is synthesized continually regardless of growth conditions.

Inducible enzyme:

1) An enzyme that is synthesized *only* in the presence of the substrate that acts as an inducer.

2) An enzyme which is present in trace quantities within a cell but whose concentration increases dramatically in the presence of substrate molecules

Enzyme B is clearly inducible since it is normally present in minute quantities and is markedly _induced_ by the presence of the substrate.

Enzyme A would fit the definition of a constitutive enzyme, since it has non-negligible activity in the absence of the substrate.

Regarding the inducible qualification, I personally feel that the definition is quite clear and the fivefold increase is not dramatic, compared to B-galactosidase increase of about 1000-fold. Therefore, I would conclude it should not be thought of as an inducible enzyme. It is clearly induced to some extent, but since it has marked residual activity and a small induced response I would not say it fits in the same box as other inducible enzymes.

To conclude, I would say the B is inducible, while A is constitutive.

Most correctly speaking: your enzyme is constitutive because it expresses all the time; and the substrate acts as an activator of your enzyme because it enhances the enzymatic activity

The situation of constitutive expression that can be enforced by induction is observed with many enzymes belonging to the cytochrome P450 superfamily and. is well understood in most cases.

For example, we detected CYP24A1 expression in different cell cultures that were essentially devoid of the natural substrates for extended time. After supplementing vitamin D metabolites, the expression of CYP24A1 rose up to 10 000 fold within few hours and declined rapidly to basal levels after having metabolized the substrates.

See...no system in biology follows an all or none phenomena...its always relative....so when u explain the results...do it accordingly..

You and other contributors as M. Repic are right to say (better I would say) that A seems to be constitutive and B inducible, but a lot of intriguing questions are around. Why do you know that there are two different enzymes? Is enzyme A michaelian? If so, it is logical that addition of exogenous substrate increase activity 5-fold (specific activity or just reaction rate because saturation?). What you mean with excess of substrate?. Is your system similar to, for instance hepatic hexokinase (enzyme A) and glucokinase (enzyme B)?. Is B appearing in the presence of an inhibitor of protein synthesis, such as cycloheximide or cloramphenicol (depending the system)?

It seems you have a nice system, and the point of constitutive vs. inducible is only one of the interesting things to explore. Go ahead and good luck

Dear Foster,

The first enzyme "A" definitely an Constitutive Enzyme because it is expressed constantly. I am not sure the increase in activity in presence of substrate is due to higher gene expression or increase in substrate concentration. If the increase in activity is because of the increased expression of that specific gene, it can be designated as inducible. For confirmation you have to do the kinetic and expression studies.

The second enzyme "B" may be termed as a PRODUCT-INDUCIBLE Enzyme because of the elevated expression rate after the presence of "excess substrate".

Upon my understanding constitutive enzyme concentration will remain same at normal physiological condition. The variations in substrate concentration can not alter the expression rate. But some physiological needs can alter the rate of such enzymes by activation or inactivation.

Please go through the following article

Nitrate Reductase as a Product-Inducible Enzyme

Drora KAPLAN, Nurit ROTH-BEJERANO, and Herman LIPS

Eur. J. Biochem. 49, 393-398 (1974)

I think only measuring the activity of enzymes A and B you won't be able to conclude if you have induction. I agree with others, I would apply other techniques as transcriptional fusions or RT-PCR, associated with western blot. This will clarify if you have a metabolic control of the activity dependent of substrate availability or a mechanism of transcriptional (or translational) regulation.

How do you measure the activity of enzyme A in the absence of the substrate?

Most of the "noise" you can read in the answers comes from the lack o definition in your question.

1) Your explanation start with "expressed all the time". Now that relates (and I have to do some guessing) to gene expression+protein synthesis. It does not mean it is fully active. A protein can be synthesized in an inactive or partially active form and its activity can be changed by an activation mechanism, or viceversa.

2) You add the substrate (I assume you are taking of providing external substrate to the cell) and the activity increases. Now I do not know if you refer to increase in measurable enzyme activity in a crude extract, to the enzyme activity as deduced from something you measure in the whole cell, or to the amount of enzyme protein as measured with some other method.

Reading all of the above answers, I guess you have figured out yourself an answer to your question. But for the sake of sanity to this intellectual exercise, I think most of us would appreciate a clarification to the true question.

Best wishes,

Rogelio

There nothing wrong in calling A as induced enzyme. All induced enzymes are produced constitutively at a basal level. There is no definite limit what should be the level of synthesis. The extend of synthesis depends on the efficiency of repressor on the enzyme under consideration. There may not be any very tight-fit regulation over the synthesis of the enzyme. Another important study you may carry is to find how glucose regulate production of A. If induction process operate glucose would inhibit stimulation of production by inducer

Thank you for all the replies, they helped confirm my belief that it is a constitutive enzyme. The question arose as a reviewer stated that it cant be a constitutive enzyme as activity has increased. This got me doubting my own interpretation of what defines constitutive enzyme.

My apologies for the lack of details in the question, but I was unable to give specifics about the enzyme in question. Unfortunately RT-PCR/western blot was not performed as I agree this would help define the enzyme.

The study was essentially measuring specific activity under several growth conditions ie growth with and without the substrate. This was measured in cell lysate and normalizied against total protein. Separating out this activity by suicide inhibitors I then purified the individual enzymes, identifying them protemically. Its a nice system which integrates into a degradation pathway.

Anyway thankyou again, hopefully in a couple of months you will be able to read all the details.

Alec