The skin's acid coating: SFVAMC researchers explain its origin and how it maintains skin integrity

By Kevin Boyd on August 15, 2001

Your skin is coated with acid. While that might sound disturbing, the mild
acidity of the skin’s surface actually helps to maintain the strength and
cohesiveness of the skin.  Now researchers at SFVAMC have discovered where this
acidity comes from, and they suggest how it may help to hold the skin
together. 

The findings, published in the latest issue of the Journal of Investigative
Dermatology, may be used to develop therapies for skin problems such as
psoriasis. 

For decades scientists have known that the outer layers of the skin are about
as acidic as a bowl of crushed tomatoes. This acidity was believed to help ward
off infections by preventing the growth of bacteria.  But the source of all
this acid and how it helped to maintain the skin’s strength and integrity,
remained a mystery.

In their latest study, Joachim Fluhr, MD, a postdoctoral research fellow at the
SFVAMC and UCSF, and his colleagues lead by SFVAMC dermatologist Peter Elias,
MD, a UCSF professor of dermatology, tested the hypothesis that the acid is
produced when enzymes break down fat-like molecules in skin cells, called
phospholipids, into smaller acid-tipped fat molecules called fatty acids.

To test this theory, the researchers used a hairless breed of mice and treated
patches of their skin with a chemical that blocks the conversion of
phospholipids to fatty acids.  They then observed what effect this treatment
had on the skin. 

The treated skin quickly lost its acidity, and this change also had a negative
effect on the skin’s integrity, Fluhr said.  Treated patches of skin were more
susceptible to evaporation and drying than untreated skin, he said. 

Furthermore, the reduced acidity also made the skin less cohesive.  When the
researchers repeatedly applied strips of adhesive tape to the skin, they found
that significantly more protein stuck to the tape from the inhibitor treated
skin than from normal skin. 

“We have shown clearly that the skin is generating the acid as it converts
phospholipids into fatty acids, one of the natural steps in the formation of
the skin barrier.  Blocking this conversion has a marked effect on the acidity
as well as the skin’s integrity and cohesiveness,” Fluhr said. 

Other experiments suggested a possible explanation for how acidity might help
to preserve the skin’s integrity. By examining skin samples under a confocal
microscope, they found that skin treated with inhibitor drugs had fewer
desmosomes, which act like staples to fasten skin cells to one another.

“The protease enzymes that break apart these desmosomes are sensitive to the
pH, or acidity, of the skin. So it makes sense that when the pH becomes more
acidic, these enzymes are activated to break apart the desmosomes, allowing
skin cells to be shed more easily,” said Elias.

The findings in the study could aid the development of drugs to treat diseases
in which the skin’s acid production is out of balance.  One example might be
psoriasis, in which skin cells are too cohesive and pile up on the skin’s
surface, Fluhr said.  “It may be that by artificially adjusting the skin’s
surface acidity, we can optimize its barrier function and promote improvements
in skin health,” he said. 

Co-authors on this study included: Jack Kao, SFVAMC dermatology researcher;
Sung Ahn, MD; Kenneth Feingold, MD, SFVAMC physician and UCSF professor of
medicine; and Mahendra Jain, professor of chemistry and biochemistry,
University of Delaware.

The San Francisco Veterans Affairs Medical Center has been a primary affiliate
of University of California, San Francisco since 1974.  The UCSF School of
Medicine and the SFVAMC collaborate to provide education and training programs
for medical students and residents at SFVAMC.  SFVAMC maintains full
responsibility for patient care and facility management of the medical center. 
Physicians at SFVAMC are employed by the Department of Veterans Affairs and
also hold UCSF faculty appointments.