Prolactin molecules-its types and its assay technique & clinical significance

 

Q.1. What is prolactin?? Prolactin (PRL) is a globular protein synthesized and secreted by Lactotrophs in the anterior pituitary gland .

Q.2: What are the three kinds of PRL molecules??  Ans;-Three major variants of PRL can be found in the blood: monomeric PRL (monoPRL), big PRL (bigPRL), and macroprolactin (macroPRL).

A) The monomeric form has a molecular weight of 23 kDa and accounts for most of the total PRL immunoreactivity in the serum of both normal subjects and those patients with hyperprolactinemia.

B) BigPRL has a molecular weight of 48–56 kDa and is thought to be a covalently bound dimer of PRL, accounting for 10–15% of PRL immunoreactivity.

C) MacroPRL has a molecular weight of 150–204 kDa and consists of an antigen-antibody complex of monoPRL and IgG.

Macroprolactin is a large, heterogeneous form of prolactin with limited bioavailability. Detection of macroprolactin by different immunoassays varies widely. It is obligatory to determine the immunoreactivity of macroprolactin by the Ortho Clinical Diagnostics Vitros ECi prolactin immunoassay, establish the most effective method for interpreting the prolactin concentration after “PEG-precipitation:- and correlate the clinical features of hyperprolactinemia with the presence of macroprolactin.

 

 

 

Q,3; Half life  and different bioactivity of  three types PRL molecules ? Ans; While the half-life of monoPRL ranges from 26–47 minutes macroPRL has substantially longer renal clearance resulting in its accumulation in the serum. Owing to its large size, macroPRL is thought to be confined to the vasculature with limited bioavailability to PRL receptors. However, this remains controversial, as some studies report that patients with a high concentration of macroPRL exhibit signs or symptoms of hyperprolactinemia such as galactorrhea, menstrual irregularities, or infertility .

Q. 4: Which kind of Lab tets is most relevant and informative to detect active monomeric PRL?? Ans;-The gold standard method for detecting macroPRL is gel filtration chromatography (GFC), a procedure that allows for quantification of all three variants of PRL.  Free prolactin is  monomeric prolactin, with reference intervals derived from PEG-treated samples.

Q.6: Where from PRL is synthesized?? In humans, prolactin is produced both in the anterior pituitary  and in a range of sites elsewhere in the body. Lactotroph cells in the pituitary gland produce prolactin, where it is stored and then released into the bloodstream. The normal values for prolactin are: Men: less than 20 ng/mL (425 µg/L) Nonpregnant women: less than 25 ng/mL (25 µg/L) Pregnant women: 80 to 400 ng/mL (80 to 400 µg/L)

 

 

 

 

 

Q.8” Take home meassage:--To distinguish "true" hyperprolactinaemia from apparent hyperprolactinaemia that is caused by raised macroprolactin content . The Electrochemilusenec Immunoassay PRL after PEG precipitation (monomeric) should be employed  and we should refer our patients to that lab which posses that facility.

Q.10 .What is PEG ppt method –how we the clinicians should interpret??

By PEG ppt when the Recovery ratio of about > 60% mostly monomeric.

But if recovery rate if between 40-60% then sample contains fair amount of macomolecule and oligomeric PRL molecules.

Then one has to proceed for gel filtration chromatography

If recovery rate is < 40% then mostly macromolecules.

The Vitros ECi prolactin immunoassay detects macroprolactin. PEG-precipitation is an acceptable surrogate to detect hyperprolactinemia in the presence of macroprolactin when using a prolactin reference interval derived from PEG-precipitated reference sera. Although testing for macroprolactin should not substitute for standard evaluation of hyperprolactinemia, identification of macroprolactin may clarify a diagnosis and direct appropriate therapy.

Other studies report that macroprolactinemia cannot be differentiated from hyperPRL based on clinical symptoms alone .

 

Because several of the signs and symptoms of hyperprolactinemia are non-specific, it is possible that the occurrence of symptoms with macroprolactinemia is coincidental and that the two are not causally related .

From a clinical perspective, it is important to identify the presence of macroPRL as the cause of hyperprolactinemia in order to avoid inappropriate treatment with dopamine agonists or radiological investigations This is particularly important given the caveat that 5–10% of healthy individuals have a pituitary anomaly that could be interpreted as an adenoma ,Macroprolactinemia has been found to occur in 15–46% of hyperprolactinemic specimens and its identification could reduce unnecessary treatment as well as the number of idiopathic cases.

The immunoreactivity of commercially available PRL immunoassays vary widely in their detection of macroPRL .One study of nine different immunoassays demonstrated 2.3–7.8-fold differences in measured PRL concentrations in 10 sera containing predominantly macroPRL

This suggests that assay variability is likely due to different combinations of capture and detection antibodies used in the various PRL immunoassays. While most of the commonly used immunoassays have been well-characterized with respect to macroPRL immunoreactivity, there is only a one report describing the Vitros ECi (Ortho Clinical Diagnostics, Raritan, NJ) PRL immunoassay .

In that report, a single specimen demonstrated moderate crossreactivity with macroPRL.

 [3]. Unfortunately, this method is labor-intensive and not suitable for performance in clinical laboratories. In contrast, precipitation with polyethylene glycol (PEG) is a widely used screening test for macroPRL and is easily performed in clinical laboratories. A low PRL recovery after PEG treatment indicates the presence of macroPRL. This method has been validated against GFC [16, 17] and recoveries <30–50% have been used as the thresholds for the detection of macroprolactinemia [4, 8, 18]. Unfortunately, reliance on a relative percentage makes this method subject to potential misinterpretation because low recoveries have been observed in patients with increased amounts of both monoPRL and macroPRL [3]. In consideration of this, a more rigorous definition of macroprolactinemia has been advocated requiring that concentrations of residual PRL (after removal of macroPRL) fall in the range of sera from a healthy reference population similarly treated with PEG [8]. Another suggested approach is to estimate monoPRL from the post-PEG PRL concentration from a regression equation derived from a correlation between PRL after PEG-precipitation and monoPRL determined by GFC [19]. This approach would compensate for the loss of monoPRL during PEG-precipitation and permit the use of an established reference interval PRL. There are no published studies that have compared these different methods for interpreting the residual PRL concentration after PEG-precipitation.